Hi, i'm new on this forum and after searching and looking for answers for my problem wihtout luck i'm now asking directly.
here it goes.
I have reacently bougth an old cigarette 21 with an 454 mercruiser mid 90's engine and bravo one drive.
It has an enourmous powerloss and is chugging on the left side exhaust, constantly and when reving over half throttle it backfires and shoots up the carburator.
what i've don until now.
1. changed sparkplugs
2. compressiontested it (on cold engine so it's not the real readings i know but it was about 120 psi on all cyl. so nothing really popped out as an answer there...)
3. tried several distributors and set the timing.
4. changed the carburator. edelbrock 1410 (the old one was cracked anyway)

The previous owner said that he reacently had the heads overhauled with new valves and sealings. because of some burned valves.

i really hopes that somebody can give me som clues of what's going on......

sorry for poor english

Svein Ronningen a Norwegian in need

A couple of initial thoughts;

1. when you set the timing was it steady or bouncing around?
2. are you sure you set the timing correctly and to correct setting?
3. Could be as simple as crossed or bad spark plug wires. Did you validate the firing order and insure right plugs wires in right socket of distributor and on right spark plug?
4. make sure the valves were set correctly. Pull off valve covers and reset rocker arm preload
5. if all the above checks out could be very lean which would cause backfire through the carburetor? Try richening up the carburetor.

Along with the above suggestions, make sure no valves are hung up in their guides and check for proper rocker movement (could have wiped some lobes off the camshaft).

I have a 1994 Merc. Mag. Had similar issues. Carb, get it re-done. and change the coil. I bet a case of heiny. it's the coil.

Also be sure to start at tdc on #1 and make sure rotor is sitting correct. Then check firing order.

It could be a lifter problem or the lifter not moving freely in the lifter bore.

120psi on ALL cylinders tells me everything in the valve train is working . Popping back through the carb and the ignition timing was checked , leads me to diagnose a cam missing some lobes . I doubt it would show that kind of compression numbers with a valve hanging open .

thanks for answering.

1. when i tried to set the timing i could'nt really set it correctly, when it got down to 10* and close to 8* it just dropped down and the scale was not possible to read.
but it was chuggin in the left exhaust nomather what the timing was, i also took of one and one wire on the left side but the symptoms where the same.
2.i re routed the wires just to be sure, and i used the firing order on the intake manifold, which was the same as the order on the distributor cap.
3. all the plugwires are also new.
4. i'l try to adjust the valves tomorrow, i was told to loosen on rocker at the tim until it starts to make noise and then tighten til it's quiet again and a quarter after that. Is that corect?
5. the carburator is brand new and opened for inspection. it is an Edelbrock 1410 750 cfm, identical to the originale weber that was on before.
6. the cyl's weren't exactly 120psi more like 115-124 psi all eavenly spread in that range.
7. i have not changed the coil my self but the previous owner said that he did with no luck.
8. can the distributor have been put down at the wrong spot by the previous owner?

Thanks again

Svein Rønningen

Answer to # 4 . Not really the correct way but will work .
Answer to 1 and 8 . Most definitely yes . Bring number 1 piston to top dead center on compression cycle and verify that your distributor rotor is pointing at #1 spark plug wire in your cap . If not , pull distributor and adjust . Pull #1 spark plug and place finger over hole , bump engine over a little at a time till you feel compression pressure blow on your finger , then set on TDC . Easy way to find compression stroke .

what type of ignition module you have on engine ?
if you have thunderbolt 5 , to set up timing you need to ground purple-white cable (two locations possible for that cable , by the thermostat housing or by distributor )