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Hot Running Problem – Twin Stromberg

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I have a hot running issue with my 1973 GT6 that has been getting worse over the past 50 or so miles. Car starts and runs fine but when it gets right up to temperature it starts to lose power and eventually stalls. After a two or three minutes wait the car will start back up again, run for another few minutes and then lose power and stall out again. Other than replacing the distributor, I have eliminated ignition as the source of the problem by swapping out old components for known good ones. I am left thinking that this is a fuel related problem. I have a new fuel pump that I can try at some point. I have rebuilt plain CD carburetors (many years ago now) but have never pulled CDSEV carburetors apart. I have a Haynes manual to hand and feel confident that, with care and patience, I can strip them down and rebuild them. However, I was hoping someone might have some magic insight about a potential source of the problem that will have a quicker or less dramatic solution. Any thoughts? Sticking temperature compensator? Damaged diaphragm? Mark
 
Hi Mark. The temp comp has but a minor affect. It is a "mini-choke". It opens an air path to the mixing chamber when the set temperature is reached. If it does not open your carb will run very slightly rich, but it is not significant except at idle speed. And, what you describe does not really sound like a symptom of a leak in the diaphram, either.
I might suggest that you try this. Make the same drive as you have been, but try leaving the fuel cap open or loose.
If the seal of the cap has gotten air tight that may cause a problem with fuel delivery to the pump.
 
Most problems are electrical; so you need to be sure you've eliminated all those potentials before messing around with the carbs. It sounds like something breaking down in the ignition, like a bad condenser (very likely culprit), bad points gap, bad coil etc.
If it really is on the fuel side then it sounds like maybe a vapor lock (not running headers are you?) or a fuel float/needle valve problem, you can rough check that fuel is delivered to the carbs by pulling off the delivery hose at the carb after it quits, careful you don't start a fire there though!
Have you pulled the plugs to check whether the mixture is right? If it's starving for fuel it would likely show as a lean mixture on the plugs ie white plug ends.
 
Ignition is definitely OK. I’m running a Pertronix igniter system and have swapped out both the igniter and coil for new ones with no difference (my first thought was the coil breaking down.) Inlet/exhaust is standard. I have not pulled the plugs recently, so that’s a good prompt.

Likewise, I’ll check to see if the gas tank is venting correctly. Now that I think about it, I have an in-line filter just before the fuel pump that I have not checked.

Would a fuel float/needle valve problem on one carburetor cause me to lose all power?

This does feel like fuel starvation. The problem happens after the car has been running for a while and gotten hot, but ambient temperatures around here (Atlanta, Georgia) have not been high enough for me to think this is a simple vapor lock issue.

There is still a lot of emissions related plumbing that could be causing a problem. The anti-overrun value has been removed, so this is not an issue. Might a culprit be a blocked charcoal canister?

It will probably be next weekend before I have time to do so more troubleshooting. I’ll report back with findings thereafter, but keep the ideas coming in the meantime.

Thanks.

Mark
 
Re: Hot Running Problem – Twin Stromberg

This is strange.

Are you sure your carburetors are CDSEV?

The CDSEV had a thermostatic choke.
If your car's chokes are operated by the choke cables, it's a CDSE (since you say it has the temperature compensator).

It's still a strange issue. I would have suspected ignition system also.

If you want to test the fuel pump, get one of those little square electrical low-pressure pumps & splice it in with flexible rubber fuel line between the exit of the fuel filter & the inlet for the metal fuel "yoke" at the two carburetors. During the test, close off the lines to the disconnected fuel pump by stickin short pieces of fuel line with bolts stuck in the ends of them & secured with little hose clamps.
 
Re: Hot Running Problem – Twin Stromberg

CDSE is correct. My error earlier. Thanks for the fuel pump test tip. It is certainly is a puzzling problem. I'll sleuth around some more this this weekend and report back on progress. Mark
 
Re: Hot Running Problem – Twin Stromberg

A better way, IMO, to check for fuel pressure problems is to get a vacuum test gauge that can also read fuel pressure https://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350012122401
and tee it into the fuel line near the carbs. That will point out any kind of problem, including blocked tank vents, lines that mysteriously block & unblock, pumps that put out too much or too little pressure, etc.

Use a long hose to the gauge and prop it under a wiper blade where you can see it while taking a test drive.

A blocked carbon canister can definitely cause drivability problems, tho usually they only happen at idle.

BTW, I have a PDF of the Triumph emissions control service training notes from 1973. PM me if you'd like a copy (about 8 Mb).
 
Re: Hot Running Problem – Twin Stromberg

Unless you've already done so, pull a couple of your spark plugs and read them. A lean or rich condition can be the root of many temp-related problems.

Let us know . . .
 
Re: Hot Running Problem – Twin Stromberg

I had this problem back in 2005, which just happens to be the last year I have driven my car, but I digress. My problem turned out to be with the carburators and the needle valve. I pulled the bottom of the carbs, cleaned everything really well, especially the needle valves, set the float, reassembled and I was back in business! As for the reason I haven't driven since 2005 is due to a frame off restoration... but this is the year!!! (I hope!)
 
GT6_Mark said:
Would a fuel float/needle valve problem on one carburetor cause me to lose all power?
Click to expand...
Sure will; when I had one of my ZS carbs set with the float level too low it would intermittently fuel starve the rear 3 cylinders and the car could barely get out of it's own way when that happened. I tracked it down by reading the plug colors.
 
I asked by race engineering buddies about this issue today and they suggested fuel valves/floats would be a good place to start. Key seems to be the gradual loss of power pointing to fuel starvation of some sort. I have a game plan for this weekend. Plan A is pull the bottom off the carbs and check/clean/reset and rebuild as suggested. If this does not work, Plan B is more sleuthing by rigging the low pressure electric fuel pump I have scavenged to run the car so that I can check the flow rate of the engine mounted mechanical pump. If the rate is good, I'll try and check pressure and see where that gets me. Look out for a report early next week. Mark
 
If none of the above suggestions pan out, check your fuel lines. I had the same symptoms, tried pretty much everything listed here, and it was a fuel line that would clog up once everything was warm and running, but unclog after sitting overnight. It was a royal pain.

GT6s are nice...

Good luck.
 
I had a simular problem on a volvo p1800, a piece of trash in the gas tank would get sucket to the outlet pipe and block the fuel line, if I sat for a few minutes the trash would move and back in business, had to drain and vacuum out the gas tank to finally solve the problem
 
Hondo Your a lucky fella,read about a guy doing what you did and not only did he clean his tank, he also cleaned his lower intestinal tract and I believe the vacuum cleaner is still in orbit.Gas fumes and electric motors =BIG BANG!!!
 
This is what I did on my GT6 this weekend: Checked spark plugs. Cleaned out all fuel and vapor lines. Removed then cleaned float bowls. Removed then cleaned float needle valves. Checked/set floats for correct operation/height. Blew out all jets. Refitted. Drove 15 miles in mixed traffic.

This is what I discovered: Spark plugs were good with no distinction in condition or color front to back. Fuel and vapor lines seemed clear (did not get to take the gas tank out, but plan to clean out the tank later.) Float bowl on front carb was noticeably drier than the rear, and the float needle valve did not seem to be acting as smoothly/easily as I would have liked. The float needle valve on the rear has obviously been replaced at some stage as it was a different, slightly heavier, design (and nice and bright and shiny) and worked easily. Closed float heights were just to spec., but the front was slightly lower than the rear.

There was no definitive "eureka" moment. However, the drier float bowl and slightly poorer action of the needle valve on the front carb leads me to think that this needle valve was sticking. I'll be driving the car this week when it is dry. This should give plenty of opportunities to see if I have a cure or just got lucky on my first few runs out.

This is the first time I have messed with the carbs on this car, and I am reminded how easy the Herald/Vitesse/Spitfire/GT6 cars are to work on for this kind of issue.

It has also been a good introduction to the CDSE carbs. While I did not touch any of the tuning or emissions elements, I have got over my initial concerns that these were significantly different to the CDs and SUs I have toyed with in the past.

I'll fit a replacement for the suspect float needle valve. At the same time, I'll tinker with the balance as I measured this to be a little out. That is, assuming I have a cure. If the problem persists, I'll rethink and come back for other ideas.

Thanks all.

Mark
 
Update and a real result.

Despite my earlier work and the thought that I might have found the problem in a sticking needle valve, my fuel starvation problem returned last weekend and did not go away after resting for a few minutes like all previuos occassions.

This time I was able to check fuel pump delivery at the carbs. Nothing when cranked, although I could see gas in the in-line fuel filter ahead of the fuel pump. Disconnected the fuel line ahead of the filter and gas was barely seeping out. It took three or four good shots of air before the gas tank would bubble up like it was boiling. Eureka! Good old blocked fuel line.

I took the gas tank out today, cleaned it and the fuel line thoroughly. There was not a huge amount of debris, but obviously enough to block the line from time to time. Now gas runs easily from the fuel line ahead of the filter.

One of the things I did find out was a number of mismatched hoses and fittings. I have managed to correct all but one. I’ll post separately for ideas on the latter.

Thanks for everyone’s input.

Mark
 
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