QuestionHoward,
i have a 75 tr6 151 cu.in. 6cyl and it will not run right
it has webber down draft carbs and elec ign conv. which has been replaced twice the problem is it backfires threw the carbs and if you hold it at 2000rpm it sounds like a popcorn machine i am at my wits end I just can't pin this thing down I change the timimg it helps a little but not really if you have any ideas please help
AnswerHi Nick,
When you have a problem like this you need to start from scratch as we had to do in a shop.
All piston engines require the same thing to run and run correctly.
Compression, fire & fuel. (with provisions on each).
First you must do a "dry" and a "wet" compression test.
"Dry" first, be sure the engine turns over 4 or 5 times and throttle wide open. Then put about a tablespoon of engine oil in the spark plug hole and run the same test again. You should not see more than 10 to 15% difference between the two tests. Do all 6 cylinders the same way and give me the test results.
If you have any cylinders that are low on either test, check the valve clearance. (.010 in.) unless the sticker is still under the hood and says otherwise. (The sticker under the hood always overrides any manual)
Set the ignition timing at 10 deg. before TDC (static).
PROVIDING EVERYTHING UP TO THIS POINT IS CORRECT, start the engine and run it up to the point that it runs the worst. If that is 2000 RPM you must set it there by adjusting the throttle stop screws (not by hand)
If you have rubber covers on the spark plug wires you will need an ice pick or such with a wire clipped to the metal to the engine ground and probe each plug to short that cylinder out while noting the RPM change. Do the same test at idle speed. DO NOT PULL THE PLUG WIRE OFF EACH PLUG TO DO THIS TEST AS YOU WILL DESTROY YOUR ELECTRONIC IGNITION DOING IT.
This is called a cylinder kill test and you are looking for one or more cylinders that are not pulling their load. Write down the results and let me know on both kill teats.
Last test is fuel. Again set the throttle screws to set the RPM to the worst RPM and take a can of something like WD-40 or Brake Clean and spray it into the air intake and note the reaction. You are not looking for it to smooth out although it may but you are looking for an RPM change. Does it speed up or slow down? Let me know the results of this test.
This method of testing divides the engine into sections and puts the problem in usually one section which narrows the problem to a smaller area.
I rarely know what is wrong but with this method of testing I NEVER fail to find the problem using this method.
let me know the results of these tests and if it has not found the problem we will go to the next set of tests.
Howard
