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Discussion Starter · #1 ·
Hi all, I’ve recently installed a brand new pair of Weber 34ICTs on my otherwise stock 1500 single port engine (1969 Beetle). Got it running pretty well following the advice of Wayoutwestie’s very good info on his site, and it’s driving fairly smoothly now. Just did my first long journey (250 miles), and after a quick inspection on arrival I found that the left hand carb was really hot to touch, but the right hand one was cool. Any ideas where to start with this? Is it normal? The mixture screws are in the same position, and the linkage (CSP bell crank) opens both carbs to full throttle and (mostly) comes back to rest both on their idle screws. Anything else I’m missing?

Thanks in advance!

Andy
 

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Carb is heated by the head.
So you need to find out if the 3/4 cylinder head is running hotter than the 1/2 side. Have you got an infra red temp gun? Check the head from underneath and compare side to side. Is it running smoothly / evenly? All the tin in place, including underneath?
 

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Discussion Starter · #3 ·
Carb is heated by the head.
So you need to find out if the 3/4 cylinder head is running hotter than the 1/2 side. Have you got an infra red temp gun? Check the head from underneath and compare side to side. Is it running smoothly / evenly? All the tin in place, including underneath?
Thanks for your quick response, I haven’t got a gun, but all the tin is in place and it’s running pretty well. What are the things that could cause one head to run hot? I’m thinking carb (mixture too lean), valve clearances, but presumably not timing as that would affect both sides wouldn’t it?
 

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Yes, I’d agree with that thinking.

Given it’s a pre-doghouse fan shroud, I imagine 3/4 side does run hotter but I couldn’t say whether it should be noticeable at the carb.
 

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It should not be really hot. 1 idea.... is there any standard flaps fitted in the housing and is the flaps on the hot side open fully when it's running
 

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Discussion Starter · #6 ·
It should not be really hot. 1 idea.... is there any standard flaps fitted in the housing and is the flaps on the hot side open fully when it's running
Just checked and I can just about see that the flaps are doing their job on that side. I have just looked at the valve clearances and seen that no. 3 exhaust is tight, so that will probably be the cause! Thanks both.
 

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Discussion Starter · #7 ·
It should not be really hot. 1 idea.... is there any standard flaps fitted in the housing and is the flaps on the hot side open fully when it's running
Just checked and I can just about see that the flaps are doing their job on that side. I have just looked at the valve clearances and seen that no. 3 exhaust is tight, so that will probably be the cause! Thanks both.
 

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If the exhaust valve is tight, that is a sign that IT is stretching. Beware of the dreaded dropped exhaust valve.
 

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Discussion Starter · #9 ·
If the exhaust valve is tight, that is a sign that IT is stretching. Beware of the dreaded dropped exhaust valve.
Okay, after a proper check, I’ve realised that the valve clearances are actually within a thou of where they should be (was checking the wrong cylinder!). FWIW I’ve set them all to 6 thou. Any other ideas what it could be. I’m thinking it can only really be mixture, as it’s only affecting one side. But both carbs are set the same!
 

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Could be air leak. If it is, it will rev up and not want to come down until some seconds have passed. Have another look at the flaps will ta. Partially blocked main maybe. Look at the plug colour on the hot side. Lack of fuel will make a plug look white on the porcelain. Pull each lead while it's running. The idle quality has to noticeably reduce on each plug lead removal. 1 at a time.
 

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Okay, after a proper check, I’ve realised that the valve clearances are actually within a thou of where they should be (was checking the wrong cylinder!). FWIW I’ve set them all to 6 thou. Any other ideas what it could be. I’m thinking it can only really be mixture, as it’s only affecting one side. But both carbs are set the same!
Did you tune the idle mixture based on a specific number of turns, or tune by ear?

Air leak is a good shout.
Also testing the “Rev drop” when pulling the plug lead (loosen at the dissy cap & pull one at a time with pliers)
 

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Discussion Starter · #13 ·
Did you tune the idle mixture based on a specific number of turns, or tune by ear?

Air leak is a good shout.
Also testing the “Rev drop” when pulling the plug lead (loosen at the dissy cap & pull one at a time with pliers)
I tuned it by ear; started with both at 2.5 turns out, then turning both in half turns until the engine stumbles, then back out a half turn.

What am I checking by pulling the spark leads off each cylinder, just that all are firing? They definitely are, I can tell just by listening. Anything else to be determined by doing that?

I’m going to take the vacuum nozzles and block-offs out of the manifolds and put some PTFE tape on the threads, in case air is leaking there. Can’t do it until I’m back home though.
 

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Verify that both main jets are the same number. Check for rag stuck in back of fan.
 

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By removing one HT lead at a time, you are checking how much that cylinder is contributing. If one lead is removed and the idle rpm doesn’t change it’s not doing anything useful. You can also use it see more subtle differences (big drop is doing more work & small drop is doing less). Down side is it doesn’t tell you why. Might be out of balance carbs, or failing spark plug or leaking valve. Etc
 

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Have you checked at the bottom of the manifolds for air leaks? SP manifolds are prone to working loose and leaking.

Also how are you regulating your fuel supply?
 
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