'72 250 Zeniths (INAT 32/40's) - Hot Start Problem

[rgnprof]

I have the '72 year model Zeniths - INAT 32/40 with no vent valve and inside/internal ventilation only. When I read about hot start problems all I seem to come across are vent valve issues.

Any other reasons?

I have to floor the accelarator and crank the engine for several seconds before it starts, so I am reasonably sure that I have a 'vapor-boil over' problem occuring when I shut the car off, which is causing it to flood. I just don't know why its occuring. Absolutely no problems with cold start, BTW.

ryan

[todds]

How are your exhaust manifold heat risers? The big round weights under the carbs... they should either rotate or be wedged in the "off" (up) position.

[rgnprof]

My exhaust manifold butterflies are welded in the 'closed' position, meaning that I have the flaps welded shut, limiting the exhaust heat to the carb bases.

ryan

[230/8]

Ryan:

Are you certain the manifold heat valves are open, i.e. diverting hot gas away from the carb base? The open position is with the weights lifted up and off the rest stop on the manifold, i.e. rolling them up toward the engine block.

If they are truly open, then you should look at fuel pressure, engine operating temperature, and float level, and fuel return valve function to ensure all are in order. You should also ensure the metal heat shields are still in place sandwiched between the two thick carb base insulators.

230/8

[rgnprof]

This whole open/closed issue with the heat risers is confusing - and I know it depends on WHAT you are talking about being open or closed!

Since I had the manifolds off, when I say closed I am referring to the actual flaps themselves in the exhaust manifold. I welded those flaps in the CLOSED position, which is blocking off the exhaust gases from reaching the carb bases.


As far as the other mentioned items - I have never measured fuel pressure. I already mentioned in another thread that my engine seems to be running a little hotter than normal, at least since I last changed out the thermostat and the water pump (couple of years ago??).
I have double and triple checked the float levels but I still could be off - would this particular problem be due to too high or too low float level - too high, right?
As far as I can tell the fuel return valve - which is vacuum operated by a small triangular shaped valve on the front carb - seems to be working right, but I'm not really sure how to check it???
Heat shields are in place.

ryan

[todds]

I am having heat soak issues as well with my BRAND NEW ($$$) webers on an m180 in a very airy fintail engine compartment with an engine that runs the same correct temp all the time. It didn't occur with the zeniths before the conversion. The webers called for removal of the fuel return, as they do not include a return valve/orifice and line fitting. So far I have changed the metal inline fuel filter before the stock fuel pump with a plastic one. I have also re-routed the fuel lines to the opposite side of the webers (away from the valve cover) and it has made *somewhat* of an improvement. The webers are such that the fuel inlet can be screwed in either facing the engine side of the carb, or the fender side. The zeniths already have the inlets on the fender side. This could be directly related to what is happening in your car (although for different reasons)

I have also received a Carter p4070 regulated 4-6psi rotary vane fuel pump which is known to work well for webers from jegs which I plan to try to lower heat and improve the flow of fuel through the lines. This should keep the fuel pressure steady, quiet and happy and lower the chances of vaporization.

First you have to determine if the fuel is boiling in the lines or the carb or both. Which one do you think it is and why? If I had to guess, I'd look at the fuel return line and pump as well, although when my Zeniths had a bad fuel return valve they tended to dump raw gas down the carb throat throwing off their operation entirely. I'm not sure what can be done to lower incidence of carb float bowl boiling besides the aforementioned heat risers, spacers, and sheilds at the carb base. I don't think the Zeniths are prone specifically to boil their gas in the bowl, in fact based on my experiences the webers are moreso. If this is happening, your engine/underhood temps are probably way too hot. You said it is very hot and humid where you live.

[230/8]

Ryan:

OK, so let's assume your check-outs have eliminated the previously mentioned items as culprits in the problem.

Less obvious, but no less important:

1. Have you verified your timing against specifications...not the idle speed setting but the timing value at 4,500 rpm? This is an example of my long-held belief that if you are paying attention you can learn something new every day. I have timed my own 230/8 for many years at idle speed using the 4 degree value mentioned in all the manuals. After getting a factory TDM, I noticed something in the small print: the 4 degree value is set at cranking speed, not idle speed...two different animals. I then set the timing to 37 degrees at 4,500 rpm, according to the book, (never did that before) and then checked it at idle speed of 900 rpm, all w/o vacuum. The timing is actually 7 degrees at idle speed. Interestingly enough, my own heating problems have abated substantially with the extra 3 degrees; who woulda thunk it?

2. Are you still using the ignition vacuum retard system? You might try it without to see if this assists your temperature issues in a way that reduces the fuel boiling issue. If you have a dual-diaphragm distributor make sure the vacuum line stays attached to the advance side, you can plug the one to the retard side. My car runs fine with the distributor vacuum hooked up like its European bretheren, yours might, too.

3. some over heating may have to be tolerated. I have not eliminated all of mine, but it is not a continuous source of trouble, and the fuel seldom boils, except when it is blistering hot, but then these are old cars with carburettors. Add in AC and your cooling system is operating at its limits.

230/8

[Chas H]

You could try lowering the float level a mm or 2. IIRC there should be insulating spacers under the carbs-are these present?

[rgnprof]

When you bring up the timing issues, that opens another can of worms, so to speak. I am currently running with no vacuum to the distributor box and timing set to 8 BTDC (which WAS set at idle). I do not have functioning speed relays (Crane ignition) and I only have manifold vacuum on these carbs, so it doesn't really make sense to attach the vacuum advance - it just speeds up my idle. I may fool with trying to set up the retard - an old post by Arthur Dalton tried to convince me several years ago that this would be better, but I just couldn't make it work.

Anyway, I will check the timing at 3000 and 4500 rpms - according to my manuals the timing should advance 42-50 degress at 3000 rpm and 46-54 degrees at 4500 rpms, but that is WITH vacuum. Since my manual says I should get 8-12 degrees vacuum advance at 4500 rpm, then I should be looking to set the timing at 4500 rpm to somewhere between 34 and 42 degrees, WITHOUT vacuum? Right?

I will check the fuel return set-up and the floats again, the insulators are in place.

ryan

[230/8]

About 37 degrees at 4,500 rpm w/o vacuum. Check the vacuum port at the bottom of the front carb to see if it has a signal at idle speed. If not, it is venturi vacuum. If it is weak, it may stlll be an OK source. Also, check your distributor; you may be able to fit a single port vacuum cannister that will work with the vacuum signal you have. A stronger diaphragm needs a stronger vacuum signal to operate. Usually a manifold vacuum source comes off the manifold or the base of the carb, but a port at the base of the carb does not automatically mean it is manifold vacuum.

230/8

[rgnprof]

This car only has manifold vacuum - at idle the vacuum from both the front and rear carbs measures at 15-17 psi, drops immediately when you open the throttle and then increases with engine speed.

Prior to my rebuild, I have fiddled with this timing and vacuum issue ad nauseum. The 2 best suggestions that I have found are to pull retard vacuum to get the idle down and to help the off idle stumble (which I haven't been able to do), or just disconnect the vacuum sources totally - sacrificing any vacuum advance and just adjusting the timing to 8-10 degrees BTDC, avoiding backfiring, etc. This has GENERALLY been my approach...

ryan

[todds]

Would an earlier, simpler (perhaps more appropriately curved) distributor fit?

[rgnprof]

I'm pretty sure I have read of others doing that - I just don't have specifics, like which distributor to use, nor do I have any lying around. It might be worth trying, if I could find the appropriate distributor.

ryan

[Blue 72 250]

I don't think it is a timing issue. Did you verify the choke positions? How is the ballance between the two? I know that you are running a little hot after the rebuild, as to be expected. What about the plugs - are they looking clean after the rebuild? I am just tossing out the obvious issues that I can think of.

[todds]

As far as your off-idle stumble is concerned, are your accel pumps squirting gas? Is the stumble better or worse if you unplug your chokes or otherwise force them to stay almost closed? This way you can tell if it is a rich or lean stumble, it might help point you in the right direction. You can also tell by the condition of the plugs assuming it's fuel delivery. First you have to narrow down the three major problem areas to one is it A) cooling B) fuel delivery or C) ignition.