Carb or Ignition issue?

Prophead · Post by **Prophead** » Sat Aug 13, 2016 10:28 pm

Roll on acceleration, I get a hesitation after putting my foot down either from steady cruise or if I lift off and go again. Its most pronounced after a downshift, however if I blip the throttle when downshifting I don't get the hesitation.

Initially I thought this might be to do with the carb throttle pump setting (Edelbrock 1404 Weber CFM500) but after recently cleaning the dizzy cap electrodes and rotor the hesitation is not so pronounced, so now I'm not sure which avenue to go down.

Hints, tips and suggestions please.

stevieturbo · Post by **stevieturbo** » Sun Aug 14, 2016 8:18 am

Could be either or really and anywhere in between, but I'd guess most likely carb tuning
Especially if it's on fast throttle movements, ie when accel enrichment is most active.

sidecar · Post by **sidecar** » Sun Aug 14, 2016 8:27 pm

Start be cleaning the carb out, by cleaning it I mean totally stripping it down including removing the boosters and cleaning the fuel wells and emulsion tubes. I have seen this issue when the air bleeds in the primary boosters become blocked. They are small vertical brass tubes that poke out of the top on the boosters, with the boosters removed you should be able to see daylight when you look up through the air bleeds.

Prophead · Post by **Prophead** » Mon Aug 15, 2016 5:29 pm

sidecar wrote:Start be cleaning the carb out, by cleaning it I mean totally stripping it down including removing the boosters and cleaning the fuel wells and emulsion tubes. I have seen this issue when the air bleeds in the primary boosters become blocked. They are small vertical brass tubes that poke out of the top on the boosters, with the boosters removed you should be able to see daylight when you look up through the air bleeds.

Thanks. I'll break out the owners manual diagrams and look into this.

sidecar · Post by **sidecar** » Mon Aug 15, 2016 5:41 pm

Prophead wrote:
sidecar wrote:Start be cleaning the carb out, by cleaning it I mean totally stripping it down including removing the boosters and cleaning the fuel wells and emulsion tubes. I have seen this issue when the air bleeds in the primary boosters become blocked. They are small vertical brass tubes that poke out of the top on the boosters, with the boosters removed you should be able to see daylight when you look up through the air bleeds.
Thanks. I'll break out the owners manual diagrams and look into this.

If you are very careful you can remove everything without ripping the gaskets which means that you can use them all again. You can end up swapping the primary boosters over which means that the top of the carb won't fit back on, easy to spot when you know about it!

Prophead · Post by **Prophead** » Mon Aug 15, 2016 7:26 pm

If you are very careful you can remove everything without ripping the gaskets which means that you can use them all again. You can end up swapping the primary boosters over which means that the top of the carb won't fit back on, easy to spot when you know about it![/quote]

I'll follow the advice of Mr China and take a bunch of photos so I know where it all goes when it comes to putting it back together.

I have Wynns carb cleaner, anything else I need for the cleaning job? Unfortunately I don't have airline.

sidecar · Post by **sidecar** » Mon Aug 15, 2016 9:19 pm

If you don't have compressed air then just try and blast every hole with the cleaner which a presume is in an aerosol can. I'm not kidding about the air bleeds, god knows how they get blocked but if they are your carb will run like a bag-o-shite!

Prophead · Post by **Prophead** » Tue Aug 16, 2016 4:08 pm

Think I might have found the problem. The fuel in passage way that fills the float chambers had a lit of rubber / silicone bits in it which had clogged the little filters. The rest of the carb was pretty grubby as well. Sprayed the cleaner through every jet, hole ect I could find.

The main gasket that seals the carb to the intake had silicone sealant applied is this required? I have a replacement for this gasket and have cleaned both mating surfaces, should I apply sealant or leave it dry?

sidecar · Post by **sidecar** » Tue Aug 16, 2016 4:57 pm

Prophead wrote:Think I might have found the problem. The fuel in passage way that fills the float chambers had a lit of rubber / silicone bits in it which had clogged the little filters. The rest of the carb was pretty grubby as well. Sprayed the cleaner through every jet, hole ect I could find.

The main gasket that seals the carb to the intake had silicone sealant applied is this required? I have a replacement for this gasket and have cleaned both mating surfaces, should I apply sealant or leave it dry?

Don't use silicone sealent anywhere in the fuel system, it is not petrol proof, it turns into a sticky jelly! Just leave the gaskets faces dry. The top of the carb is above the fuel level anyway so it certainly does not need anything other than a gaskets and clean mating surfaces.

Prophead · Post by **Prophead** » Tue Aug 16, 2016 5:05 pm

sidecar wrote:
Prophead wrote:Think I might have found the problem. The fuel in passage way that fills the float chambers had a lit of rubber / silicone bits in it which had clogged the little filters. The rest of the carb was pretty grubby as well. Sprayed the cleaner through every jet, hole ect I could find.

The main gasket that seals the carb to the intake had silicone sealant applied is this required? I have a replacement for this gasket and have cleaned both mating surfaces, should I apply sealant or leave it dry?
Don't use silicone sealent anywhere in the fuel system, it is not petrol proof, it turns into a sticky jelly! Just leave the gaskets faces dry. The top of the carb is above the fuel level anyway so it certainly does not need anything other than a gaskets and clean mating surfaces.

I figured as much, and you are right it had turned to a sticky jelly. All cleaned off now.

Frankly I am amazed the car was running at all

I'll put it back on the car now then and see how we get on.

Sidecar - thanks for all the tips and advice....no doubt I will be back for more.

Prophead · Post by **Prophead** » Tue Aug 16, 2016 7:42 pm

Well, the carb is back on the car and the car is running again. I wont know if this has cured the problem or not until I get it back out on the road. However, it seems to start a lot easier, especially from hot; and, I had to reset the idle as it had increased by 200rpm post cleaning which seems a positive thing.

The throttle response seemed a lot crisper and more responsive (all be it not under load) until the engine and carb were at operating temp then the hesitation was back but only in the lower rpm range ie blipping the throttle from idle up to about 2000rpm, blipping the throttle above 2000rpm it was fine.

I tried all 3 position on the throttle accelerator pump arm, doesn't like the top one but runs, bottom position kills it completely, so I guess I will be sticking with the default middle position.

I managed to read the numbers on the primary main jet and metering rods
Main Jet Primary .086
Metering Rod 6552 which I believe reads as .065 x .52

Looking at the owners manual it would seem this is the default setup from the factory.

sidecar · Post by **sidecar** » Tue Aug 16, 2016 8:01 pm

The settings that are you using are the defaults, I have worked on loads of these carbs, they always need re-calibrating. Now loads of people will disagree with my last statement because they plonked the carb on their engine and it runs just fine. Really the only way to know if it runs just fine is to analyize the exhaust gases in all the modes of operation of the carb, the AFR values for cruise, acceleration and WOT are well known.

I don't like giving out carb settings because really it is impossible to jet a carb over the web but you could try a 67-55 rod, this is 8% leaner on cruise and 7% leaner on acceleration. This setting has worked on some 3.5 engines that I've worked on. (Assuming that you have a 3.5 here!)

I am also assuming that there is nothing wrong with your setup such as the choke being stuck on or the air filter lid being too close the the top of the carb or the fuel pressure being too high.

Prophead · Post by **Prophead** » Wed Aug 17, 2016 7:13 am

Thanks again Sidecar. I'll hold of re calibrating until I'm sure the fundamentals are correct. Trying to find a 'carb' specialist in Scotland to do the fine tuning.

Yes - it is a 3.5 with CR 9.75:1 which may or may not have had head work and cam. It is on Edelbrock manifold and aircleaner (see pics). I've had the car a couple of months, bought as running driving but unfinished project. I'm gradually working through the issues.

I've replaced the knackered fuel pump and put a high flow filter in the pressure regulator which is a Malpassi Filter King 85mm. Fuel pressure set at 5.5psi.

Couple of other things I noticed whilst doing the clean yesterday (See pics)
1. it appears that the choke flap has been removed
2. A section of the top front lip of the carb has been cut out
3. The engine has a slight front to back slope but the carb appears to lean forward

https://www.flickr.com/photos/144556909 ... res/9572q7

PS photos taken before I cleaned the carb

sidecar · Post by **sidecar** » Wed Aug 17, 2016 8:42 am

The mod to remove the choke plate support looks a bit 'brutal'

I have done this sort of mod on a few carbs, I don't just cut a slot out but remove all of the support and blend it in at the radius. Anyway the mod to your carb will still help the primary side of the carb breath better which is good! (Having said that it may just be possible that the way it has been cut is upsetting the air flow into the primary side of the carb and that is causing your issue, I doubt it though!)

The fact that your carb slopes forward is 'normal', a lot of engines are setup like this. I found that it can cause a rich stumble after breaking quite hard. Believe it or not fitting a wedge plate to get the carb level does reduce this problem a lot!. The other issue with these carbs is a rich mixture when cornering hard, raising the floats about 3mm higher than the 'book' setting reduces the fuel height which solves the problem.

When you cleaned the carb out did you take a look at the air bleeds on the primary boosters? If you hold the booster up to the light you should be able to see light through the air bleed. (The air bleed is the small brass tube poking up out of each booster)

Reading your post again, it does sound like its a lean stumble as it gets far worse when the accelerator pump is on the outer hole of the pump arm, this is the weakest pump shot, this surprises me a little as at first I thought your issue was a rich stumble because it gets worse as the engine gets hot.

What colour is the exhaust smoke when you blip the throttle?

Maybe someone has fitted the weakest piston springs which is holding the carb in its weak mode, i.e. the rods are not lifting at all. take a look at the springs under the pistons that lift the rods, the standard ones are orange which will probably be OK-ish. The orange paint falls off the springs so you may need to look very hard to spot any traces of paint! The springs have not been cut short have they?

Prophead · Post by **Prophead** » Wed Aug 17, 2016 9:06 am

sidecar wrote:The mod to remove the choke plate support looks a bit 'brutal'

I have done this sort of mod on a few carbs, I don't just cut a slot out but remove all of the support and blend it in at the radius. Anyway the mod to your carb will still help the primary side of the carb breath better which is good! (Having said that it may just be possible that the way it has been cut is upsetting the air flow into the primary side of the carb and that is causing your issue, I doubt it though!)

The fact that your carb slopes forward is 'normal', a lot of engines are setup like this. I found that it can cause a rich stumble after breaking quite hard. Believe it or not fitting a wedge plate to get the carb level does reduce this problem a lot!. The other issue with these carbs is a rich mixture when cornering hard, raising the floats about 3mm higher than the 'book' setting reduces the fuel height which solves the problem.

When you cleaned the carb out did you take a look at the air bleeds on the primary boosters? If you hold the booster up to the light you should be able to see light through the air bleed. (The air bleed is the small brass tube poking up out of each booster)

Like many things on this car, the ideas and component selection have been very good but the finer points of modification, installation and setup are some what lacking.

I checked the airbleeds, blasted carb cleaner through them so am pretty sure they are clear.

I may be suffering the hard braking stumble so might look into the wedge plate option. Dont recall having any issue with hard cornering when I did the test session at the circuit. That might change when I get sticky tyres on though.