Any suggestions to what's causing a bad misfire?

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veedweeb
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Any suggestions to what's causing a bad misfire?

Post by veedweeb » 4th May 2022 - 9:40am

I was driving down to Standford Hall on Saturday when my '72 Square suddenly developed a really severe misfire after about 100 miles or so of an absolutely faultless drive.

It's mechanically stock except it now has twin weber 34 ICTS and a Huco electric fuel pump. The ICTs have been rejetted and balanced. Immediately before the misfire started the car was a joy to drive - smooth, quiet and responsive.

After about 100 miles, I was pulling out of a junction and there was really bad clutch judder. Immediately the car started misfiring and had virtually no power. It didn't misfire with VERY light throttle but any attempt at accelerating or driving up a hill was met with a severe misfire. Even pulling away from a standstill is very difficult

To try and fix it, I have:

- ensured the carbs are seated tightly and are sealed to the manifolds
- ensured the manifolds themselves are tight to the heads
- checked the valves for operation and gaps
- fitted brand new: Coil, points, condenser, rotor arm, distributor cap, HT leads and spark plugs (everything correctly gapped and adjusted etc)
- Balance pipe between the carbs is all good
- Vacuum lines to the DVDA distributor are all good

The car starts and idles perfectly, hot or cold.

I haven't checked the ignition timing yet, but I know the distributor hasn't moved - it still has the stock fuel pump in place on the engine even though it's not connected (I'm waiting for a blanking plate to be delivered) and the vacuum canister on the distributor is touching the side of the fuel pump, just like it was when I set the timing. Plus it starts and idles so well that I can't imagine it's the timing.

The only other thing is that when I pull the vacuum line off the carb and suck on it, I can see the base plate in the distributor move which it should to advance the timing, but when I pull the line off the manifold and suck it, there is no resistance and the distributor doesn't move to retard the timing.

I have got a brand new canister to fit so I'll do that and check/reset the timing, but I wondered if anyone else had any other suggestions?

Thanks in advance
Steve.

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1972 Squareback
1963 Turbo WBX powered Beetle
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Rob 400e
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Re: Any suggestions to what's causing a bad misfire?

Post by Rob 400e » 4th May 2022 - 5:36pm

Seems you have covered all the basis already. Nothing more annoying I have been in your shoes many times. I’m no mechanic but have a thought, could it be the fuel tank breather clogged? This would result in vacuuming. Worth a check.
Type 411 Le four door auto .

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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 4th May 2022 - 5:55pm

Rob 400e wrote:
4th May 2022 - 5:36pm
Seems you have covered all the basis already. Nothing more annoying I have been in your shoes many times. I’m no mechanic but have a thought, could it be the fuel tank breather clogged? This would result in vacuuming. Worth a check.
I hadn't considered that, but it's definitely worth checking out. Thanks 😊
Steve.

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Rob 400e
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Re: Any suggestions to what's causing a bad misfire?

Post by Rob 400e » 5th May 2022 - 4:10pm

If you have not run the vehicle prior to driving to Stanford hall you may of sucked up some grunge from the tank. I’m guessing that could be a big job to clear!
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 6th May 2022 - 9:03am

Rob 400e wrote:
5th May 2022 - 4:10pm
If you have not run the vehicle prior to driving to Stanford hall you may of sucked up some grunge from the tank. I’m guessing that could be a big job to clear!
It's possible - I'm not ruling anything out! But I think it's unlikely.

I fitted the electric fuel pump about a week ago, and there was already a fairly new fuel filter under the tank. I fitted a second filter immediately before the fuel pump (did it without realising the one under the tank was there) and then I discovered that the fuel line from the pump is 8mm and the one to the T piece on the carbs is 5.5mm so I needed a reducer. I've got a stash of fuel filters with dual sized barbs on them, so I used one as a reducer: 8mm in, 5.5mm out.
So this car has actually got 3 new fuel filters on it.

I popped the fuel filler cap off and there was no vacuum. Even with the cap completely off, the misfire is still there so it's not a clogged breather either.
Steve.

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scottm
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Re: Any suggestions to what's causing a bad misfire?

Post by scottm » 6th May 2022 - 11:19am

Just a guess , i had something similar with a Triumph - is the advance mechanism working correctly in the dizzy ? I know you said the vacuum works but check the mechanicals are free to move .
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 6th May 2022 - 1:15pm

scottm wrote:
6th May 2022 - 11:19am
Just a guess , i had something similar with a Triumph - is the advance mechanism working correctly in the dizzy ? I know you said the vacuum works but check the mechanicals are free to move .
It's a double vacuum distributor. One hose goes to the carb for advance, and another goes to the manifold for retard.

The advance seems to be OK. If I suck on the vacuum line, the base plate of the distributor moves freely enough, but the retard hose doesn't move the plate and there's no resistance to me sucking on it so I suspect the vacuum can is dud. I swapped it last night to a brand new single can. The advance works and the retard is now blanked off, but it's no better at all.

I'm gonna pull the carbs off and double check everything with them next, I think.
Steve.

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broady_6
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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 8th May 2022 - 3:35pm

Sounds almost certainly like youve got a fueling issue. As soon as you open the throttle more air rushes in, but in your case more fuel doesnt, the mixture goes weak and you get a mistfire. Have you checked which exhaust branches are getting warm? This should help tell you which cylinder(s) are missing. Then you can suspect its one carb or the other.
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 9th May 2022 - 10:46am

broady_6 wrote:
8th May 2022 - 3:35pm
Sounds almost certainly like youve got a fueling issue. As soon as you open the throttle more air rushes in, but in your case more fuel doesnt, the mixture goes weak and you get a mistfire. Have you checked which exhaust branches are getting warm? This should help tell you which cylinder(s) are missing. Then you can suspect its one carb or the other.
I have discovered that the previous owner who swapped the stock carbs to the ICTs used the stock manifolds, which is not causing the misfire but it is causing clearance problems for the air filters so I'll get a set of the correct short ICT manifolds and fit them. At the same time I'll strip the carbs and make sure they're all OK and set up/balanced correctly.

After that, I'm out of ideas!
Steve.

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broady_6
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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 9th May 2022 - 3:51pm

I think itll be more a case of a tiny piece of dirt blocking a jet, rather than set up/balanced correctly.
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 9th May 2022 - 4:51pm

broady_6 wrote:
9th May 2022 - 3:51pm
I think itll be more a case of a tiny piece of dirt blocking a jet, rather than set up/balanced correctly.
I think you're right - especially because it was running well for 100 miles and then just started misfiring.

However, I don't like the way the balance pipe between the carbs is installed (My fault - I did it but I don't like it), plus the manifolds are wrong and I noticed that the crossbar linkage could be better too so I'll go through the whole lot and sort it all out properly.
Steve.

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broady_6
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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 9th May 2022 - 8:28pm

:thumbsup: Keep us posted :D
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 12th May 2022 - 8:40am

Small update.

When the engine is idling, pulling the plug leads off one a time to see if one of them isn't firing reveals that they are all firing fine.

I've put my carb balancer tool on and found that one was reading just over 10 and the other was reading less than 5. The numbers are arbitrary, but they should both be the same so I rebalanced them and adjusted the linkage so that's all good. It definitely idles even smoother now but the misfire is still there.

My new manifolds should be arriving today so I'll throw them on, making sure that the bottoms of the carbs sit flat on them. I've had problems on a previous car where the bottom face of the carb was a bit warped and was sucking air in where there was a gap between it and the manifold.

The manifolds weren't cheap at £150, but they should solve a couple of other issues as well - they'll improve and tidy up the current balance pipe arrangement, sort out the current problem of having a spare vacuum takeoff point and hopefully they are shorter too, to provide a bit more clearance for the air filters
Steve.

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Re: Any suggestions to what's causing a bad misfire?

Post by s.unsworth » 20th May 2022 - 12:53am

Try another New Condenser. Even brand new ones can be faulty out of the box these days. Had a similar intermittent Misfire. Could not diagnose it at all. Finally replaced the already new Condenser with another new Condenser. Instant Fix !

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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 13th June 2022 - 4:57pm

Ongoing update...

The new correct CB performance manifolds are shorter than the stock ones but are also wider - they push the carbs about 50mm further apart which means that the linkage I have got needed adjusting quite a bit.

I fitted them, sorted out the balance pipe and as I was modifying the linkage I noticed that the LHS carb was leaking so I took it off and apart and that's when I discovered
broady_6 wrote:
9th May 2022 - 3:51pm
a tiny piece of dirt blocking a jet
Cleaned out and put back together with the leak sorted too, but I haven't finished sorting the linkage yet so it's untested but it does look like Broady called it.

With all my fannying about, I balanced the carbs but of course now the jets are clear the balancing is out so I'll have to redo that as well.

Here are some pics of the differences in the manifolds for reference:

Image

Image

Image
Steve.

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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 18th June 2022 - 10:35am

Fingers crossed for you!
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 22nd June 2022 - 11:14am

Fixed it!

Just to recap - the misfire gave all the symptoms of a bad condenser so because the rest of the ignition system was unknown as well, I changed everything: Points, coil, rotor and cap, HT leads, condenser and spark plugs. Then I checked the timing and made sure the valve clearances were all OK.

Next up was a suspected vacuum leak and I didn't like the way the balance pipe was set up, plus I found a vacuum leak on the distributor can on the retard side.
It was at this point that I realised it had stock inlet manifolds with the ICTs on top of them which explains why the air filter was crushed against the underside of the boot floor.

A pair of new manifolds solved a few problems - they're shorter for more air filter clearance, they have a simpler arrangement for the balance tube and they don't have the vacuum take off for the distributor retard. The downside is that they also push the carbs farther apart so I had to rethink the throttle linkage. Luckily the plates that mount on the carbs can be fitted in 2 positions so it was just a case of relocating them.

It was at this point that I discovered some crap blocking a jet in the LHS carb so I stripped them both and cleaned them out.

Then I put it all back to together and went through the whole set up and balancing procedure from scratch and it's running better than ever now!

The combination of a completely new ignition system, along with zero vacuum leaks and properly balanced carbs has done the trick. It will now pull away from standstill in 3rd gear with no problems. It couldn't even do it in 1st before!

Thanks for the input, everyone. I got there in the end!
Steve.

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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 26th June 2022 - 7:31pm

Glad to be of service, enjoy the sunshine!
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 3rd August 2022 - 9:15am

Just to add a bit more to the ongoing saga....

Clearing out the blocked jet sorted out the really bad misfire but after driving it for a little while, it still wasn't right so I went through the basics again.
The new manifolds make access to number 3 spark plug really difficult and I could feel it starting to crossthread when I put it back, so that along with the leaky two-piece pushrod tubes was enough for me to pull the engine out again to make sure the plug was seated correctly.
That's when I discovered the exhaust was loose at the heads and also the head itself to cyls 3 & 4 was loose!

I pulled the heads off to replace the pushrod tubes and on cleaning everything up, I also discovered it's not the original engine and this one has had bigger barrels fitted so it's actually a 1641 now.

Anyway, it's all back together and seems to be good for now :nod:
Steve.

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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 5th August 2022 - 11:04am

I spoke too soon!!

Wednesday night I was meant to be going to a local meet, but I only got a couple of miles before I gave up and turned back.

On the motorway at very light throttle cruising (50-60mph) it was surging really badly. It would accelerate smoothly when I put my foot down, but just wouldn't cruise nicely. I think it's a bit lean, so I pulled off the motorway and looked at the engine to richen the carbs a bit and I found it had sucked the cooling bellows into the fan.
I took the bellows off completely and limped the couple of miles home and put it away. I think the time has come to get it to a specialist to set the carbs up properly now.
Steve.

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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 5th August 2022 - 7:36pm

How on earth did it do that!? Did you forget to put the clips on? hopefully it hasnt over heat the engine, as it sounds like you had actually got to the route of the problem in your previous post.
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veedweeb
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 8th August 2022 - 9:45am

broady_6 wrote:
5th August 2022 - 7:36pm
How on earth did it do that!? Did you forget to put the clips on? hopefully it hasnt over heat the engine, as it sounds like you had actually got to the route of the problem in your previous post.
I didn't have the original clip so I made something up that I thought would work but I clearly underestimated the sucking power of the mighty Type 3!

I was sure I'd got the running problems sorted. It starts instantly one the key every time hot or cold and pulls strongly and smoothly when accelerating pretty much from any speed. It's just at constant, very light throttle when cruising on the motorway that it does this surging thing. I'm certain it's a carb setup issue, but I don't see how it can be the jet sizing that's causing it because it didn't do this before. There's an old school motorsports and rally prep place only a few miles from me and they have a good reputation for tuning carbs so I'll just take it up there and they can sort it for me.
Steve.

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Re: Any suggestions to what's causing a bad misfire?

Post by broady_6 » 10th August 2022 - 7:50am

Does it need someone else to work on the car? If youve got it running ok, new bellows and proper clips. As long as you didnt over heat it, it will be as good as before!
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Re: Any suggestions to what's causing a bad misfire?

Post by veedweeb » 10th August 2022 - 9:15am

broady_6 wrote:
10th August 2022 - 7:50am
Does it need someone else to work on the car? If youve got it running ok, new bellows and proper clips. As long as you didnt over heat it, it will be as good as before!
It definitely would have got hot, but I don't think it's actually overheated. But although it's starting and running well enough, it's just not right. It fires up instantly even from being left a few days and will idle nicely all day long. It'll accelerate really smoothly too but it just won't drive nicely at light throttle. Like at 60mph on the motorway where there's barely any pressure on the pedal it surges really badly, but if I put my foot down it will accelerate up to 80+ really strongly and smoothly.

It's booked in to RE Performance about 6 miles away from my house next week so they can tune it on their rolling road. They're proper old school classic car rally prep experts so I'm confident they know what they're doing.
Steve.

Club member 2268

1972 Squareback
1963 Turbo WBX powered Beetle
2000 Mexican Beetle 'Sedan Clasico'

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