Twin 40's, but only one is working?

Had my TSR 8v running on twin weber 40's for a while now. The timing and carbs were set up by ear and it seemed to run ok although I didnt have anything to compare it to. Took it to a rolling road carb specialist today, and the timing was advanced by approx 20 degrees. And it became apparent that only one carb was doing the grunt work. At idle you could pull 2 of the HT leads and there was no difference

He sprayed carb cleaner around the inlet and one of the four carb to mani gaskets must be leaking becuase it caused the engine to bog down and almost cut out. He said the air leak is effecting the fuel air mix and stopping enough fuel getting int ote engine. The plugs of the two cylinders that werent firing were working but covered in soot and not at all wet.

He checked the jets and they werent blocked, and there was fuel in the carb.

Hes advised i replace the carb to man i gaskets again and dont do them so tight to avoid any air leaks, but im not convinced this is the entire problem, as i had air leaks on the other carb which works ok and it just made it idle higher. Why would spraying carb cleaner into a leaking gasket cause the engine to slow down rather than speed up

Ive ordered new gaskets and am hoping to go back there this week as I am going to VW action this weekend So any advice or possible problems would be much appreciated!

 

Someone please help me

 

Depending on the type of o ring plates you have fitted there could be damage of the single moulded type, or an oring may have been spat out due to a misfire. Both these scenarios will result in an air leak, which means that the carb will draw air through the leak, rather than mixture from the carb. This means that spraying something volatile like carb cleaner will have an effect on the engine running.

Sounds like a strip down of the carbs from the inlet manifold to check, then rebuilt and setup are in order.

 

Thinking about this logically... There are a few reasons why that would happen. Namely, the first thing that comes to mind, is that if you pull the leads and nothing changes then a good place to start is the plugs. Pull them and make sure they arent fowled...

Next issue... after googling a couple things it seems that when the idle drops due to carb cleaner its a case of the carb cleaner temporarily plugging the air leak. If the idle is set with an air leak then the idle would be set pretty low... further creating an issue when the leak is plugged.

You should definitely start with replacing the gaskets. I would also replace the manifold gasket... Make sure your plugs are good... and make sure the jets are the same.

Basically start over again.

Also, if you're using the thick o-ring type gaskets you aren't supposed to synch them tight. There needs to be room for them to isolate the vibrations.

That's all I can come up with for now. Sorry man. Good luck though. Post up your progress.

 

Im using the misab plates. Originally had the thick plastic ones but they leaked too much. Ive definitely done the misab plates up too tight so have bought new ones. But when I had the thick plastic gaskets, they leaked on the carb that is working, and it just resulted in a higher idle speed. Ive fitted two types of gaskets and i find it hard to beleive that a potential air leak in one gasket in both situations resulted in two cylinders doing absolutely nothing to contribute to the running of the engine. I know this problem has not only occurred after fitting the misab plates becuase when originally fitted you could tell one carb wasnt doing much but I thought when I had someone balance the carbs they had sorted this problem.

Obviously the air leak will stop them running properly but there must be another underlying problem with the carburetor for the engine to only be running on two cylinders?

 

Plug leads on the right way?

 

Thanks for the reply, i was in the process of replying when you posted

Thanks for clearing up my confusion as to why the idle dropped rather than increased.

We pulled the plugs of the disfunctional cylinders and they were covered in black soot, but not at all wet. They did spark fine but we replaced them anyway and it made no difference.

I have no reason to beleive the jets are different and they are definitely clear, but i will have the guy check their sizes when I return to the rolling road. New misab plates are on their way, but i dont think I will replace the inlet manifold gasket. We sprayed carb cleaner on all the mani gaskets and the gasket between carb and mani on cylinder 3 was the only one that caused a reaction when doused in carb cleaner.

 

Im sure I saw the guy at the R/R swap them over when he was checking for a spark so I dont think thats the problem.
Ill double check tomorrow but im 99% sure thats not the problem

 

Thinking about it It was running fine on kjet before the carb conversion and although the timing was messed with, the leads and dizzy were never removed and were numbered so deffo not the problem

 

Yeah, I wouldnt see the jets being different. Just a suggestion....

As for the black soot... thats typically a rich condition...

Clarify for me which cylinders are running correctly... which ones are supposebly inactive... and which cylinder reacted with a higher idle and which ones bogged down... I'm starting to wonder if there's more than one issue at hand

 

Dont know what the firing order is. But if the shortest HT lead is for cylinder 1, and it continues in order to cylinder 4, then its cylinder 1 and 2 which are the problem. The carb feeding these two isnt really sucking in much air. Im not sure how well the two carbs are connected, in that one maybe more open than the other at idle. But this shouldnt make much difference when accelerating as the joining linkage is connected so the butterflys have to be opening. The misab plate for cylinder 2 has the air leak and causes the engine to bog down when soaked in carb cleaner. Originally with the old thick gaskets cylinder 4 was leaking badly and it caused a higher idle, especially when one of the carb to mani bolts fell out

 

My brain hurts too much right now too think.... gimme a little bit to figure this one out...

 

So here's the best theory I can come up with....

It sounds like the car may have been tuned with the air leak already in place. This lean condition was attempted to be corrected by adjustments on the carb. Now that you're trying to have a pro tune the car you're finding that the adjustments are out of whack and discovering the air leak. The reason I say this is that more than likely it's adjusted to the rich side to make up for the air leak. That would explain the fowled/soot covered plugs... and the reason that when you spray the carb cleaner it bogs it down....

All that leads into me just giving the same advice as earlier... Replace the gaskets and continue on with a normal tune. If the carbs are clean and the jets are close to correct you will probably solve your problem.

That's basically all I can come up with for now.. especially since I can't actually see/hear the vehicle. You were already going to replace them, so I would do that before assuming anything worse.

Hope this helps and good luck. Post up your results!

 

Right tried to make some good progress tonight but failed. I replaced the gaskets, and this time did as mr haynes instructed and tightened until the cups were touching the rubber spacers, then another 1.5 turns. Yet if u spray easy start on each of them it causes the engine to bog down. I cant win! Are they genuinely leaking? None of the old gaskets were in anyway damaged. Faces of mani and carbs were clean?...

Anyway so you can start the car with ht leads for cylidners 3 and 4 disconnected and it runs no differently. And if you spray the gaskets of 3 and 4 even with the ht leads disconnected the engine still bogs down.

Using a hose to listen to 'the sucky noise', the carb for cylinders 1 and 2 is making a beautiful burble sound and definitely doing lots of work. The carb for 3 and 4 makes a mild sucking sound, which does not increase a noticable amount even when the engine is revved. When i had the gaskets off i played with the linkage and you could see the butterfly valves opening a bit so im sure they are not the problem.

It seems to me something is restricting airflow, but i dont know what? Both carbs have the choke lever but niether are wired up. Could the choke be stuck open/closed?

 

I don't think so... the choke circuit on webers simpley dump more fuel in the mix... not exactly restrict the airflow... Not only that but they generally have a built in return "spring" to push them back to the original position.

If you obviously still have an air leak there is a possibility that the cabrs/mani could be warped... but I'm not sure.

 

I thought the surfaces may be warped, but the o-rings are flexible and should seal quite well so i dont see how they can be leaking. And the mounting hardware is designed to allow the carbs to move around a bit, and take out the vibrations so i dont think you could ever seal them 100%. I think the air leaks around the gaskets are just a scape-goat for the real issue and really arent that much of a problem...

 

Right so today I took the carb in question off the manifold and started taking a few bits off it, but wont be stripping it down until I have a plan of action/memorised the diagram in my haynes manual. The linkage definitely opens the butterflys, but when you do so fuel squirts out of one side (for cylinder 4), but not the other (for cylinder 3).

The air is pulled through the carb by the vacuum created by the piston going down and the valve opening yes? So the air should be sucked in regardless of any fuel issues, yet even with the throttle open the carbs werent sucking in any/much air. With the rocker cover off I turned the engine over by hand and there didnt seem to be any problems/stuck valves.

I can only find service kits which seem to just be new gaskets and seals, but surely my problem is more than that? What other parts would I need for a full rebuild?

To have both carbs fully rebuilt is going to cost in excess of 40, and since theyve already cost me about 400 I just cant justify spending that much. There are numerous NOS single Weber 40 carbs on ebay, and I was contemplating getting one and swapping over the jets and anything else unique to my carbs. I figured this way im more likely to cure the problem.

Are all DCOE Weber 40 carbs the same, or must I have a specific version. Mine is a 'Weber 40 181', whilst the ones on ebay are 'Weber 40 127' and 'Weber 40 4'. Would this be a problem?

 

Well it sounds like a blocked pump jet for a kick off if it is not squirting fuel out of both pump jets when the throttle is opened.
It could well be that the carb has dirt or debris in it that has blocked the jets. Idle jets are around 0.5mm so even a tiny particle results in a blocked jet.

They are easy to strip, just take care and lay everything out in a methodical fashion. Take some pics if you want to log part positions.

Was the carb interlink / balance linkage properly engaged between the two carbs?

Did both carbs have a similar amount of fuel in the float bowls when you removed them?

These are all tells to what the problem may be. An experienced eye will spot these things straight away and diagnose potential issues.

And the carb numbers should match, but there can be odd numbers sometimes depending on application.

 

Ok, I take it they are readily available? Will it say on the jet what size it is? The filter inside the carb was clean.

Yes but not very well, in that i wouldnt be suprised is the working carb was slightly more open than the other at idle. But even when revved high the airflow into the faulty carb did not increase so i cant see this being the only problem, as the valves would have to have opened at least partially.

I only removed one carb and didnt take enough off of it to know how much fuel was in the bowl.

Learn by doing right!?

So the single carbs on ebay at the moment would not be suitable then? In what way would they vary from my carbs?