Issues around idling

[LittleDuckAlex]

Hello! Long time forum lurker here. Finally created an account because I could use some advice. Apologies for all the text but I wanted to get down everything I thought was relevant.

The engine/carb:
845cc (800-05) engine with a Zenith 28 IF (V05075) carb.

The main issue:
The idle is too high, inconsistent between startups, and the revs don’t come back down fully after using the throttle for the first time.

Additional known issues:
There are a couple of things I’ve noticed that will need addressing. First, the 1.3mm jet from the oil fume rebreathing system appears to be missing, unless it’s in the elbow on the manifold, but I don’t believe it is. For now I have blocked the hose going from the manifold. Secondly, the idle mixture screw looks to broken on the end as well as being very worn higher up. Also, the brass fitting with the hole for the mixture screw is actually loose, so likely not air tight. I will replace the screw and attempt to epoxy the fitting in place so it’s air tight and can’t fall out like it has on me whilst cleaning. At the time (back in November) I wasn’t thinking properly about it and put it back in place without considering the consequences of it being loose. This isn’t causing the main issue though as I tried running with the screw out, using my finger to cover the hole, and the main issue persisted.

In the most recent run I observed the following:
980rpm fully covering the idle mixture hole, with the idle speed screw backed out all the way to the point it does nothing. Using my finger to adjust mixture I could increase the revs up to 1050rpm. Increasing the revs and letting off, it dropped down to a varying 1250-1300rpm. They don’t seem to come down until the engine is left to cool before starting again. The idle is supposed to be 700rpm for my setup.


The choke is definitely fully open, I’m not even using the cable as that just broke so I’m doing it by hand. The initial opening for the throttle is correctly set at 0.85mm. The timing is set to 6°. The carb has been removed, cleaned, and put back together with new gaskets. The oil rebreather hoses have been replaced (the 1.3mm jet was not in the old hoses). I have a new needle valve but I’ve got the old one fitted. The kit I bought didn’t come with two different size washers as some do, and I compared the height of the old one to the new one and they’re 0.5mm different, the same difference between my carb and later ones (0.84mm vs 0.89mm height from the carb body). The tolerance is 0.1mm so I decided to keep the old one in.

My thinking has been that it’s a vacuum leak, but I used a smoke leak detector and couldn’t see anywhere. I could have missed something though as the smoke comes out of the air filter above the float chamber, making it hard to see a small leak. I attempted to cover it up the best I could but even then it still came through a bit and I couldn’t see smoke coming from anywhere else.

Despite not being able to see a leak anywhere, I’m still not entirely convinced it isn’t one or multiple small ones. The carb body was pretty badly warped to the point where the base surface varied by 2mm. I sanded it down so it was flat but potentially could have done a better job. The surface where the two halves joined was also not so good and I sanded that a bit too. I just didn’t want to overdo it. It does concern me slightly with how much warping there’s been. The plug that covers the manufacturing hole above the idle mixture screw had fallen out at some point in the past and had been replaced with rubber. This had perished and fell out when I begun cleaning the carb. I’ve since sealed it with a brass disc and epoxy. Is the carb potentially a lost cause? I’m hoping not and that I can repair it sufficiently for use.

So what could be causing the issue?

Could it maybe be the needle valve? Is it worth swapping the old one out for the new, despite it then being out of tolerance?

Could it be something other than a vacuum leak or leaky needle valve?

I’m obviously not going to be able to tune things without a new mixture screw, but I’d like to know what’s causing the idle to vary and not come back down before I start taking the carb apart again and ordering more parts.

I’d appreciate any suggestions.

Alex

 

[angel]

845 engines with small carburettors had the restriction for oil vapour recirculation hoses built in the inlet mainfold, at the point where the 90 deg. pipe fits, so it's normal not to find anything in the hoses.
Check that throttle butterfly shaft does not have excessive play and that butterfly does not have side-to-side movement. Take off carburettor and with top cover removed and butterfly fully closed, place a light source on top of the venturi and check light at the other side of butterfly; there should be very little light passing around butterfly edge, having a uniform pattern. It's common for these butterflies to wear at the sides, allowing air to pass through.

 

[André4Renaults]

Depending on how far you want to go, it might be an idea to have the carburettor refurbished. Carburettor Exchange in Leighton Buzzard do a very good postal service and if you were to go down this road you'd eliminate any carburettor problems at once. Another factor might be a small irregularity on the surface of the inlet manifold, which would cause an air leak, which in turn would cause the mixture to weaken and the tick-over speed to increase.

 

[orsome]

Hello! Long time forum lurker here. Finally created an account because I could use some advice. Apologies for all the text but I wanted to get down everything I thought was relevant.

The engine/carb:
845cc (800-05) engine with a Zenith 28 IF (V05075) carb.

The main issue:
The idle is too high, inconsistent between startups, and the revs don’t come back down fully after using the throttle for the first time.

Additional known issues:
There are a couple of things I’ve noticed that will need addressing. First, the 1.3mm jet from the oil fume rebreathing system appears to be missing, unless it’s in the elbow on the manifold, but I don’t believe it is. For now I have blocked the hose going from the manifold. Secondly, the idle mixture screw looks to broken on the end as well as being very worn higher up. Also, the brass fitting with the hole for the mixture screw is actually loose, so likely not air tight. I will replace the screw and attempt to epoxy the fitting in place so it’s air tight and can’t fall out like it has on me whilst cleaning. At the time (back in November) I wasn’t thinking properly about it and put it back in place without considering the consequences of it being loose. This isn’t causing the main issue though as I tried running with the screw out, using my finger to cover the hole, and the main issue persisted.

In the most recent run I observed the following:
980rpm fully covering the idle mixture hole, with the idle speed screw backed out all the way to the point it does nothing. Using my finger to adjust mixture I could increase the revs up to 1050rpm. Increasing the revs and letting off, it dropped down to a varying 1250-1300rpm. They don’t seem to come down until the engine is left to cool before starting again. The idle is supposed to be 700rpm for my setup.


The choke is definitely fully open, I’m not even using the cable as that just broke so I’m doing it by hand. The initial opening for the throttle is correctly set at 0.85mm. The timing is set to 6°. The carb has been removed, cleaned, and put back together with new gaskets. The oil rebreather hoses have been replaced (the 1.3mm jet was not in the old hoses). I have a new needle valve but I’ve got the old one fitted. The kit I bought didn’t come with two different size washers as some do, and I compared the height of the old one to the new one and they’re 0.5mm different, the same difference between my carb and later ones (0.84mm vs 0.89mm height from the carb body). The tolerance is 0.1mm so I decided to keep the old one in.

My thinking has been that it’s a vacuum leak, but I used a smoke leak detector and couldn’t see anywhere. I could have missed something though as the smoke comes out of the air filter above the float chamber, making it hard to see a small leak. I attempted to cover it up the best I could but even then it still came through a bit and I couldn’t see smoke coming from anywhere else.

Despite not being able to see a leak anywhere, I’m still not entirely convinced it isn’t one or multiple small ones. The carb body was pretty badly warped to the point where the base surface varied by 2mm. I sanded it down so it was flat but potentially could have done a better job. The surface where the two halves joined was also not so good and I sanded that a bit too. I just didn’t want to overdo it. It does concern me slightly with how much warping there’s been. The plug that covers the manufacturing hole above the idle mixture screw had fallen out at some point in the past and had been replaced with rubber. This had perished and fell out when I begun cleaning the carb. I’ve since sealed it with a brass disc and epoxy. Is the carb potentially a lost cause? I’m hoping not and that I can repair it sufficiently for use.

So what could be causing the issue?

Could it maybe be the needle valve? Is it worth swapping the old one out for the new, despite it then being out of tolerance?

Could it be something other than a vacuum leak or leaky needle valve?

I’m obviously not going to be able to tune things without a new mixture screw, but I’d like to know what’s causing the idle to vary and not come back down before I start taking the carb apart again and ordering more parts.

I’d appreciate any suggestions.

Alex

Hi, Sorry mate, 28IF is what it is- i have the same model and seems to be more of a feature than a bug how it works. I've done everything and many times, only matter which not has been realized is sinking it to a ultra sound purifying unit.I I wonder how did you measure rpms's so precisely?
I have put a new needle valve and adjusted it according to sylvie.fr instructions, found a old original OEM spare parts set including idle mixture screw(because had doubts against tip of it), casgets and even plastic seal for shaft, have planed surfaces with sanding, have tried several different gasgets against heat fro the manifold, sanded against glass, worked with jets, put heavier return spring to get lower rpm's after tightening shafts with new seals.

 

[LittleDuckAlex]

845 engines with small carburettors had the restriction for oil vapour recirculation hoses built in the inlet mainfold, at the point where the 90 deg. pipe fits, so it's normal not to find anything in the hoses.
Check that throttle butterfly shaft does not have excessive play and that butterfly does not have side-to-side movement. Take off carburettor and with top cover removed and butterfly fully closed, place a light source on top of the venturi and check light at the other side of butterfly; there should be very little light passing around butterfly edge, having a uniform pattern. It's common for these butterflies to wear at the sides, allowing air to pass through.

Thanks. Good to know that the restrictor is in the elbow. I’ll check the throttle shaft later today. It definitely has some play but I’ll do as you say and shine a light down it to see.

 

[LittleDuckAlex]

Hi, Sorry mate, 28IF is what it is- i have the same model and seems to be more of a feature than a bug how it works. I've done everything and many times, only matter which not has been realized is sinking it to a ultra sound purifying unit.I I wonder how did you measure rpms's so precisely?
I have put a new needle valve and adjusted it according to sylvie.fr instructions, found a old original OEM spare parts set including idle mixture screw(because had doubts against tip of it), casgets and even plastic seal for shaft, have planed surfaces with sanding, have tried several different gasgets against heat fro the manifold, sanded against glass, worked with jets, put heavier return spring to get lower rpm's after tightening shafts with new seals.

I know it’s a carb that can cause a lot of headaches but the fact that the idle speed screw can’t even be used when I’ve set everything up by the book doesn’t seem right. I appreciate that the timing may want to be retarded slightly for modern fuel differences, but I wouldn’t have thought it would be that significant.

I’m using my strobe gun to get measure the rpm, It’s accurate to 10rpm. I also already ran the carb through my ultrasound with a solution specific for carbs, using compressed air to clear out the passages afterwards, and I wet sanded against a piece of glass when trying to level off the mating surfaces.

 

[LittleDuckAlex]

Okay, so I took a look at the throttle butterfly and there is a very noticeable gap/amount of play. I can see this being the issue so I’m going to replace it and the shaft, as well as the mixture screw. I’ll give an update when everything’s back together again.

 

[LittleDuckAlex]

Success! The new parts arrived last week and after slight modification I managed to get everything fitted. Yesterday evening I successfully got the car to idle at the correct speed, and after increasing the revs they come back down. It drove well too, just needs a leak in the exhaust sorting.