750 Mighty Demon hard to start

[bigblockmark]

Very temperamental on starting procedure. Runs good after 5 minutes.

This is on a SBC 383 street/strip engine in a 1991 S-10 back half 9" Ford rear end with 4 link. This truck was run at the track and on the street about 10 years ago. ran low 7's in 1/8 mile and in the 6's with nitrous.. Driver was killed in a car accident. Set since then. brought to me after a couple local gys worked on it for just the hard to start issue. I have gone true it to find the issue. I did rebuild the 750 Mighty Demon. Did not help. If I can get it to start and keep it above 1500 RPM for 5 minutes, you would not know it has a problem. Idles at 1100 RPM. You can reach in and turn the key and it will start instantly and set there and run. But that first 5 minutes is tuff to keep it from dying. It will back fire and catch on fire if you don't do it just right.

Engine has very grumpy camshaft and apparently low static compression. It had high compression pistons when it was on the track. Pistons were changed to prepare for a Procharger, but cam was not. Cranking compression is 90# on first hit and 125# on 5th hit. Vacuum at idle is 7". I think this is the problem. I have tried up to 30* initial timing and manifold vacuum on the distributor. Vacuum does not increase much at all as I increase timing. I am looking for away to help the hard starting without changing the cam or pistons. It is a fun ride as is after it gets warmed up.

[Right hand drive]

Manage to get the IFR’s drilled?

Might not be enough to solve your problem anyway. Seems with the engine combo the carb doesn’t have the flexibility to run great at temp and also run ok until up to temp. Not many do without a choke. I know when I have a good tune in mine no matter what carb on top I need to sit in the car for five or so mins working the accelerator pedal to prevent it stalling when cold. Doesn’t seem as much trouble as you are having but it’s accepted as part and parcel of what the engine is and sticking with no choke carbies. I don’t think I am telling you anything you don’t know. K&N eci could be another solution but quite an expensive one to avoid the cold start only annoyance of what you are going through now with someone else’s car.

I’m keen to hear if anyone else has ideas too.

[rgalajda]

Why not just throw a carb on with a choke and see how it starts. How much trouble could that be.

[bigblockmark]

I am used to "normal" issues with race cars, no choke and low vacuum of a race cam, as on my 582 pump gas race engine, 800" lift cam and 1250 dominator carb. This is much more frustrating. As soon as I let it get below 1500 RPM it dies so fast I can't catch it with accelerator pump shots. Attempts to restart it immediately often result in a back fire and maybe a flame, (probably from pumping the throttle/accelerator pump trying to unsuccessfully keep it running) which goes out if continue cranking with no additional fuel added. If an air filter would be on it, I think it would catch the filter element on fire. This being a Demon with no choke is a issue. It also has all the Demon difference from a Holley including the different size threads for the inlets on the bowls and it is plumbed with braided hose and AN fittings. I don't have a Holley here with a choke a AN fittings to switch over and don't know anyone locally that does. That would be a good test. Even though I found nothing wrong during the carb rebuild except dirty residue from not being used, it may still be the issue, I guess.

[bigblockmark]

Update

I finally solved the hard start/ cold idle issue on the S-10 SBC with the Mighty Demon 750. I have thought it was the combination of low compression pistons, 8.8:1 and a very aggressive camshaft. I ultrasonic cleaned and rebuilt the Demon, replacing all parts in a kit. That did not help. I then changed to a 670 vacuum secondary Holley carb I had previously rebuilt but had not used on a known good engine. Did not change anything. Both carbs drove well after warmed up and gave good full throttle response.

Next i installed a separate manual switch to isolate the ignition from the start key switch. This did not help initial start, but eliminated the back fire on restart cold. I could roll the engine over and clear out the engine of excess fuel before resting.

Gave up on it for a while. Then thought, I did not really try a known good carb on it. I borrowed a Brawler 600 vac secondary that i had sold new and installed on a pick up for a friend. He sold the truck but kept the carb. Hit the key and it started and idled fine. Does not die and is responsive. Not as good at full throttle, but it solved the problem.

Now i need to try the first 2 carbs on a known good engine I guess.

[rgalajda]

[bigblockmark]

Choke is there but not functioning. Full open. Really should not need a choke in 90 degree weather? The exhaust pipes were black at idle,turned gray after a ride. It ran good at part throttle and full throttle. For whatever reason, it likes this carb on cold start up and hated the first 2.

[Right hand drive]

[bigblockmark]

Since the brawler is not my carb, it will stay together. I may have paperwork on another one i had. i just think this one has no issues and the other 2 do. I need to check and see if the engine still has only 7" vacuum at idle.

[Right hand drive]