Carburettor overheating

[benchseat]

I am having a problem with my 1972 845cc R4. After a run, the carburettor is sufficiently hot that the fuel in the float bowl evaporates into the inlet manifold. Consequently if you try to start it after a minute or two, it won't as the float bowl is empty and the inlet manifold is flooded.

If you leave it for a few minutes until the carb has cooled down, it will start perfectly OK as soon as new fuel has been pumped into the carb with a few turns of the ignition.

I have had a look at various previous posts on the Forum and it looks as if many people have experienced this problem (car stopping, refusing to start, then starting again after a few minutes) but no-one seems to have tracked the problem down to an overheating carb. I know this to be the cause as if you remove the air filter immediately after a run you can see the hot fuel bubbling out.

The engine itself is not running hot. When the car is actually in motion presumably the draught through the radiator grille, and the cooling due to evaporation of the petrol, keep the carb sufficiently cool.

The question now is - given that it is a hot carb which is the culprit, has any reader managed to solve this problem?

My carb is a Zenith 28mm. One way round might be to fit a complete carb, manifold and vacuum-advance distributor system from a late-plate French 845cc vehicle (bigger and more sophisticated carb). My other 845cc R4 (1984 LHD) has this setup and has never given this trouble. But does anyone know another way round it?

 

[Simon P]

Has the fault just started our has it been there for a long time.

Simon

 

[malcolm]

I felt I had fuel vaporisation causing hot starting problems in Ornella but never stopped to watch what was going on. That's interesting info.

Generally the problem is heat conduction from the exhaust manifold to the carb. A lot of carbs have plastic blocks in the mounting to the manifold. I don't recall one in Ornella or it could have been a really thin one. Doubling up might help. Fairly sure the GTLs have something.

Other time I've had the problem was in the MGA. That was under bonnet temperatures going well over 100 degrees which caused it to cut out in traffic queues, but I doubt the 850cc R4s would suffer from that especially if a different carb sorts the problem.

 

[Steve Hennessey]

The GTL does have a plastic block between the carb and the manifold. It's also 28mm so this may be thicker than the one that is already on your car. The GTL does not suffer from this problem but in the winter hot air needs to be extracted from the exhaust front pipe to stop icing and a faster warm-up period. Your car may have a thin plastic insulator block which is great for faster winter warm-ups but then allows the petrol to boil in the hot weather.
Another problem could be that the fuel level in the carb is too high and it is syphoning into the venturi. It's worth checking to see if the needle valve is not leaking as well. If it's not leaking but just worn then experiment by adding a very thin washer under the needle valve to lower the fuel level.

 

[Simon P]

The only common fault we had this time of year was carbs icing up.
If you haven't moved the wing nut on the side of the carb ,our moved the air hose from the inner wing to the manifold .The water content in the petrol can ice up in the carb neck,The car will slowly loose power till in comes to a Holt,when you switch it of the heat from the manifold rises and thaw out the carb ,it then starts and runs fine till it happens again .

 

[benchseat]

Thank you for replies so far : the float and needle valve have been renewed, and the problem does not occur when the car is actually in motion (it comes on about 2-3 minutes after the engine has been stopped or has been running in slow traffic). I therefore don't think it is syphoning.

There is a plastic block (as described by Steve) already installed. In fact everything is exactly as it should be according to the exploded (!) diagram in the Parts Book.

Carburettor icing occurs in cold, humid conditions. Latent Heat Of Evaporation (of the petrol) cools the main jet to below zero Centigrade, and the moisture in the air condenses on it and blocks the hole. This can happen (I have experienced this myself) when the air temperature is well above zero. Damp is the cause. In any case this is not the problem here.

My wife has reminded me that we have had similar problems on our earlier R4s (all 845cc). It therefore looks to me as if this is a fundamental design flaw with the carbs on the earlier 845cc engines. Possibly the R4 was not designed for sustained, relatively high-speed running. It does beg the question of how, if this is a design flaw, it was circumvented in all those R4s pictured as doing African rallies in the 1960s/70s.

I suspect that the problem is radiation (off the exhaust manifold), not conduction. My manifold is pretty rusty on the outer surface and probably acts as a perfect Black Body. I'll see if I can devise some crude radiation shield and report back. In the meantime any further useful info would be welcome - thanks, gentlemen.

 

[angel]

I am very surprised to read your experience, Benchseat, as I (and a lot of other people) have been driving a setup exactly like yours (28IF on an 845 engine), often in very hot conditions (40+ Celsius outside, heavy traffic, think what is happening under the bonnet!) and have never encountered any problems.
The engine does seldomly flood if I switch it off for, say, five minutes in such conditions, but only to the extent that it needs a little more cranking than normal, with the throttle wide open, to start it again.
Your thought of heat radiation from the exhaust manifold is correct, but it is equally strange why this hadn't worried Renault engineers (and much more R4 drivers!) for so many years.

Maybe your problem is related to high fuel pressure in the line between the fuel pump and the carb, and the needle valve cannot seal correctly? I had this problem on my grey R4 (had almost the same symptoms as you, only that it would also flood when idling for long on hot days). Try adding one more (or two-there is no problem) gasket between the fuel pump and the block to lower the fuel pressure and see what happens.

 

[benchseat]

Thank you Angel - I'll look into this. Do you mean add an extra cardboard gasket (approx 1mm thick) or the plastic spacer (about 4mm)? Between the pump and the block at present I have one plastic spacer and one cardboard gasket.

 

[angel]

Yes, exactly this. If you have a fuel pressure gauge it's better (but not essential) as you can check the difference in pressure obtained.

 

[benchseat]

I have now substantially cured the problem by fitting an improvised radiation shield around the carburettor. This comprises a thin steel sheet, bent round the underside of the carb in a "U" shape and fixed to four of the five carburettor bolts (two on each side).

Although the problem still occurs when the engine is switched off, it is much reduced and the carb feels much cooler at the end of a run - also I have not had any trouble when actually on the road.

This seems to demonstrate that the "hot starting" problem is due to the carb absorbing radiant heat from the exhaust manifold. When the car is in motion the airflow through the radiator grille keeps the carb sufficiently cool (now, anyway) but when the engine is switched off the residual heat from the engine block heats the entire under-bonnet space (carb included) and causes fuel boil-off. This could well be the reason why other R4 owners have experienced similar problems (see numerous posts over the years).

The only practical consequence is that when the engine has to be restarted it is necessary to turn the key severel times in order to refill the carb bowl. I didn't follow up Angel's suggestion of excess fuel pressure as this would surely give a problem when on the road rather than only when the engine is switched off.

This rather begs the question (see Angel's post) of why everyone, particularly in hot countries, has not had a similar problem. There could be circumstances specific to my vehicle (for example the surface of the exhaust manifold is rusty and this would make it a more efficient heat radiator - possibly a new, shiny (or plated?) one would not be so bad), but the following points also occur to me :

(i) Does unleaded petrol have a lower boiling point than leaded?

(ii) Online sites give petrol as boiling between 35C and 75C. This is obviously a full range during which all the fractions (light and heavy) boil off - but is petrol for use in hotter countries (e.g. Angel's Greece) mixed so as to have an overall higher boiling point than
that for cold countries (e.g. UK)? This might explain why I have had the problem but he has not.

If any reader has a contact in the petroleum industry . . . .

 

[bison]

filter?

When touring around Normandy in the summer a very similar problem occured with Dave Grays' 845TL. The carb would flood after a longish run, when you came back to the car it smelled strongly of petrol and wouldn't start until it was cranked over for quite a while, filling the float bowl and off it would go. We traced the fault to an aftermarket fuel filter which had been fitted between the pump and the carb, it had been located at the top of the engine above the rocker cover. When the car had been switched off the heat rising from the engine expanded the vapours in the half full fuel filter increasing the pressure on the needle valve and flooding the carb. Removing the filter cured the problem. Occasionaly my TL would do the same, but I didn't have a filter fitted, the same pipe between the pump and filter had been replaced at some time and was a bit short, fitting a longer length of pipe has solved the problem in my car.
Alan.

 

[r4 gtl]

Carb overheating

My Haynes manual (in Dutch) describes this problem, as a problem of the Zenith IF28 with high temperatures after 10 to 30 min. of rest. It also suggest a modification to the fuel-line to solve the problem, after making sure the float-needle en pressure of the pump are not at fault.
The solution is to make a T-joint in the fuel-line between the carb en the pump and a T-joint in the line just before the pump. These T-joints are connected via a new piece of fuel-pipe, and one of the joints has a restrictor in it.
In this way the pressure build-up in the fuel-line through heat is released to the gastank.
The restrictor is necessary to enable the pump to build up pressure when working normally.
So apparently Renault did adress the problem!
Of course shielding of the carb will help also, or just live with the problem...

 

[benchseat]

"Angel" himself posted a diagram of this fix in February 2005. Go to the Thread entitled "Ignition Problems".

It looks as if the problems that many people have had regarding hot-starting have now been definitively tracked down to their cause, which was some deficiency in the Zenith 281 setup for 845cc vehicles - causing problems in some but not all machines. We'll just have to remember next time someone writes in . . .

Many thanks for all the help, gentlemen.

 

[angel]

By reading Bison's post I suddently remembered that for all these years that I considered normal a little harder starting after stopping for 10 minutes, I always had a fuel filter over the rocker cover. I am very curious to see what happens now, after the restoration. (I have moved the filter immediately before the fuel pump). I admit I never had this problem on the (28IF equipped) Jogging, and coincidentally, the fuel filter is located at a "cool" place. Too bad hot weather is over now...

I had almost forgotten Renault's solution with the double T junctions...it sounded interesting but I never found it necessary to resort to this, neither on mine, nor on any customers' R4.

 

[jjad]

By reading Bison's post I suddently remembered that for all these years that I considered normal a little harder starting after stopping for 10 minutes, I always had a fuel filter over the rocker cover. I am very curious to see what happens now, after the restoration. (I have moved the filter immediately before the fuel pump). I admit I never had this problem on the (28IF equipped) Jogging, and coincidentally, the fuel filter is located at a "cool" place. Too bad hot weather is over now...

I had almost forgotten Renault's solution with the double T junctions...it sounded interesting but I never found it necessary to resort to this, neither on mine, nor on any customers' R4.

@angel did you find that moving the fuel filter solved your poor hot starting issues?

 

[jjad]

I'm asking because yesterday (very hot weather), we were stuck in a traffic jam for half an hour, just moving forward slowly, then waiting. I don't think that the engine was overheating (warning light never came on, no bubbling in expansion bottle), but it would stall then sometimes be hard to start. My guess is the carb (which we we recently had cleaned) was overheating. We pulled over into the shade and waited a while, and it was fine again when we were able to drive off again.

It has always taken a while to crank over when starting from cold, which I've put down to the petrol evaporating from the carb. Perhaps moving the fuel filter (which is currently above the rocker cover), or making some sort of heat shield for the exhaust would help. Any thoughts?

Thanks

(PS: any guesses to where we stopped for a picnic on our way home?)

 

[angel]

Trying to recall most hot starting cases of the last 8 years, and no, I don't think I had real trouble at any time. At the same time I moved the fuel filter I changed the fuel pump to the earlier style that can be disassembled, and I fitted a rubber priming bulb before the fuel pump.
I can't be sure if it was the filter placement or not. In theory the carburettor float chamber is conducting heat better than a plastic fuel filter, and it's over a hotter place (exhaust manifold). But moving the fuel filter is wise, especially because it's dead easy.

 

[jjad]

Thanks angel, that's useful thank you.

 

[DAA]

@jjad did you solve your problem? Yesterday under 37⁰c and with some traffic i had the same on my 4L. I rode 40kms always at 80km/h without any problem

 

[jjad]

@DAA
I don't know for sure as it hasn't happened since. We rarely drive nowadays as we walk to work and/or work from home, and the R4 only comes out in good conditions so probably haven't been stuck for that long in traffic since then.