engine's first run

[robert leazenby]

First I want to thank all for the help I received over the last few months on my motor build. I started it last night for the first time. I had a few glitches with the rocker/lash adjustment, but after getting that straghtened out it ran like a champ. I ran it for about 20 minutes with no additional problems.

After the 20 minutes I let the idle slow a little to about 800 to 1000rpm. When I shut it down I experienced some diesel run on for several rotations with a little knocking. Upon initial start the ignition timing was about 22 degrees advanced, I set it at 12 degrees advance for the remainder of the break in. (With vacuum). The engine is built to lt-1 specs (cam, solid lifters, 11.0:1, etc)

Any ideas as to the cause of the run on. Could it be the timing not advanced far enough or possibly too much fuel. The Holley carb was set great before the engine build.

[Pat Bush]

Re: engine's first run

Robert ---

Duke and Dale are the timing guys but I will throw my two cents in here.... Given you have built a stock 70 LT-1 motor and I assume you resurfaced the heads as well, you are probably running around 11.25-1 compression. First thing I would do is boost your octane. Pump premium ain't gonna fly - you will need some Cam2 additive to get your octane up to at least 98-100 for that motor to perform to spec.

Dieseling from my understanding has to do with heat and enough heat to continue to burn gas after the engine has been shut off. Lower octane fuels generate more heat in a higher compression engine. The higher octane actually runs cooler and may solve your problem. It can be related to timing, but if you set the car to spec to start with, I would try additive or racing fuel first.

Duke and Dale will tell you to do "total advance" timing which should be about 34 degrees when mechanically fully in -- and that gives you a more accurate reading than the way the books tell you to time a car. However, I have also had good luck with the old time at idle method and that is the timing your books will state. I do not have a LT-1 or books handy right now but 12 degrees w/o vacuum should be about right for the motor providing you are running appropriate octane. Lower octane will force you to reduce timing to get the ping out and it WILL impact performance. Also, you should always time the car without vacuum advance in!

Hope this helps -

Pat

[Pat Bush]

Re: engine's first run

Robert ---

Duke and Dale are the timing guys but I will throw my two cents in here.... Given you have built a stock 70 LT-1 motor and I assume you resurfaced the heads as well, you are probably running around 11.25-1 compression. First thing I would do is boost your octane. Pump premium ain't gonna fly - you will need some Cam2 additive to get your octane up to at least 98-100 for that motor to perform to spec.

Dieseling from my understanding has to do with heat and enough heat to continue to burn gas after the engine has been shut off. Lower octane fuels generate more heat in a higher compression engine. The higher octane actually runs cooler and may solve your problem. It can be related to timing, but if you set the car to spec to start with, I would try additive or racing fuel first.

Duke and Dale will tell you to do "total advance" timing which should be about 34 degrees when mechanically fully in -- and that gives you a more accurate reading than the way the books tell you to time a car. However, I have also had good luck with the old time at idle method and that is the timing your books will state. I do not have a LT-1 or books handy right now but 12 degrees w/o vacuum should be about right for the motor providing you are running appropriate octane. Lower octane will force you to reduce timing to get the ping out and it WILL impact performance. Also, you should always time the car without vacuum advance in!

Hope this helps -

Pat

[Clem Z.]

Re: engine's first run

the run on is caused by hot spots in the combustion chambers and the carb is still suppling fuel after you shut off the ignition. the carb butterflys are open too far and fuel is being pulled through the carb. shut down the front butterflys more and if you can not get the idle speed you want try opening the secondary butterflys slightly to get the correct idle speed. you can also drill holes in the front butterflys to get the correct idle speed. you must stop the flow of fuel through the carb as the engine is coasting to a stop.

[Clem Z.]

Re: engine's first run

the run on is caused by hot spots in the combustion chambers and the carb is still suppling fuel after you shut off the ignition. the carb butterflys are open too far and fuel is being pulled through the carb. shut down the front butterflys more and if you can not get the idle speed you want try opening the secondary butterflys slightly to get the correct idle speed. you can also drill holes in the front butterflys to get the correct idle speed. you must stop the flow of fuel through the carb as the engine is coasting to a stop.

[Duke W.]

Re: engine's first run

"I set it at 12 degrees advance for the remainder of the

break in. (With vacuum)."

If you set the timing with the vacuum advance connected and let's say the vacuum advance provides 16 degrees then the initial timing is actually 12 - 16 = -4.

INITIAL TIMING MUST BE SET WITH THE VACUUM CAN DISCONNECTED unless you have TCS or "ported" vacuum advance, but even if this is your engine's configuration the vacuum can should be disconnected to insure that it does not contribute any advance. Also you should set the idle speed down as low as it will run to make sure that the centrifugal advance has not started. Initial timing is INITIAL and you must make sure that the vacuum and centrifugal do not add to it.

If my interpretation of your post is correct and you set the timing with the vacuum can connected, and if the vacuum can is communicating directly with manifold vacuum, your intial timing is actually retarded from the spec by the amount of advance contributed by the vacuum can. Timing this retarded will heat up everything in the combustion chamber and this can cause runon. Get your initial timing properly set and you should be okay.

Duke

[Duke W.]

Re: engine's first run

"I set it at 12 degrees advance for the remainder of the

break in. (With vacuum)."

If you set the timing with the vacuum advance connected and let's say the vacuum advance provides 16 degrees then the initial timing is actually 12 - 16 = -4.

INITIAL TIMING MUST BE SET WITH THE VACUUM CAN DISCONNECTED unless you have TCS or "ported" vacuum advance, but even if this is your engine's configuration the vacuum can should be disconnected to insure that it does not contribute any advance. Also you should set the idle speed down as low as it will run to make sure that the centrifugal advance has not started. Initial timing is INITIAL and you must make sure that the vacuum and centrifugal do not add to it.

If my interpretation of your post is correct and you set the timing with the vacuum can connected, and if the vacuum can is communicating directly with manifold vacuum, your intial timing is actually retarded from the spec by the amount of advance contributed by the vacuum can. Timing this retarded will heat up everything in the combustion chamber and this can cause runon. Get your initial timing properly set and you should be okay.

Duke

[Duke W.]

Re: engine's first run

Pat - there is no fundamental difference in combustion temperature with different octane fuels. Lower octane fuel might demand less timing to prevent detonation, and if timing is retarded from the optimum for each speed and load condition, the EGT will go up as will the combustion chamber boundary temperatures. Peak combustion temperature will also change with timing, but oversimplifiying the physics by saying "premium gas burns hotter", or whatever, just leads to a baseless myth.

Duke

[Duke W.]

Re: engine's first run

Pat - there is no fundamental difference in combustion temperature with different octane fuels. Lower octane fuel might demand less timing to prevent detonation, and if timing is retarded from the optimum for each speed and load condition, the EGT will go up as will the combustion chamber boundary temperatures. Peak combustion temperature will also change with timing, but oversimplifiying the physics by saying "premium gas burns hotter", or whatever, just leads to a baseless myth.

Duke

[Jack layton #896]

Re: engine's first run

Golf Tee goes in the rubber hose that provides vacuum to the distributer. The only good use I could ever find for the things! Then set the timing at the low idle speed.

[Jack layton #896]

Re: engine's first run

Golf Tee goes in the rubber hose that provides vacuum to the distributer. The only good use I could ever find for the things! Then set the timing at the low idle speed.

[Pat Bush]

Re: engine's first run

I see. Never believe what you read. Understood. Thanks for the clarification!

Pat

[Pat Bush]

Re: engine's first run

I see. Never believe what you read. Understood. Thanks for the clarification!

Pat

[Duke W.]

Re: engine's first run

Just to clarify a bit more. the peak or average combustion temperature for a given engine will be a function of the fuel's heating value, the fuel air ration and timing. I have heard reports that regular grade fuels often have a bit more heating value, but it's only about one or two percent.

Assuming no detonation and equal timing, fuels of two different octane ratings and essentially the same heating value will generate the same temperatures and burn at about the same rate, however, if the lower octane fuel detonates forcing us to reduce timing, then peak combustion temperature will be a bit less and EGT will be a bit higher. The lessened timing reduces thermal efficiency, which will create the above effects (lower peak combustion temp, more energy thrown out the exhaust), and this will show up in the real world as less torque/power and greater fuel consumption.

Duke

[Duke W.]

Re: engine's first run

Just to clarify a bit more. the peak or average combustion temperature for a given engine will be a function of the fuel's heating value, the fuel air ration and timing. I have heard reports that regular grade fuels often have a bit more heating value, but it's only about one or two percent.

Assuming no detonation and equal timing, fuels of two different octane ratings and essentially the same heating value will generate the same temperatures and burn at about the same rate, however, if the lower octane fuel detonates forcing us to reduce timing, then peak combustion temperature will be a bit less and EGT will be a bit higher. The lessened timing reduces thermal efficiency, which will create the above effects (lower peak combustion temp, more energy thrown out the exhaust), and this will show up in the real world as less torque/power and greater fuel consumption.

Duke