Dwell setting

[john cole]

does anyone know what the recommended dwell setting is for a 1987 4.3l Gm OMC engine? i still don't have a manual for this motor or boat. it is ina 1987 Mark Twain 21' not that should make much difference.

 

[Howard Sterndrive]

Re: Dwell setting

Dwell is typically 30 -35 degrees on any 6 cylinder, 4 stroke engine.

 

[john cole]

Re: Dwell setting

very good thanks.

 

[indybleck]

Re: Dwell setting

According to the clymer that I have it says for a 1987 4.3 dwell should be 39 degrees + or - 2 degrees

 

[libellav15]

Re: Dwell setting

tip - set it to the top figure as the dwell will decrease to the smaller figure as the points wear

means adjustment shouldn't be needed for a little longer

 

[bruceb58]

Re: Dwell setting

tip - set it to the top figure as the dwell will decrease to the smaller figure as the points wear

means adjustment shouldn't be needed for a little longer

Actually, dwell increases as the rubbing block wears.

 

[libellav15]

Re: Dwell setting

I stand corrected

sorry :facepalm: :redface:

 

[john cole]

Re: Dwell setting

ok so i have checked my dwell, timing, coil, points and everything else. still only getting 26 to 28 mph at 3800 rpm. the tack is with 200rpms of what the engine is doing. my prop is a 14.5 x 19. and it has slight damage to the edge's. the engine again is a 4.3l with a 2bbl carb. could it be that the boat is a bit underpowered. the engine seems to be running well now. think maybe i should down size the pitch on the prop and maybe in this case acually pick up some speed when the engine can reach its power band. i don't know what size prop came with this boat new. it is a 1987 Mark twain 210 MTD if i remember the letters right. any thoughts?

 

[JSMoore]

Re: Dwell setting

I think I replied to your question about a prop once before. I run an OMC Cobra 4.3L V6 powered 21 footer that tips the scales at 3800 lb. dry. (It's a 1987 Mark Twain MTC 215 - very similar to what you have.) My prop is a stock 15 x 17 aluminum 3-blade. I turn 4500 - 4600 RPM @ WOT & top out consistantly at 37 kts on the GPS. And, I get a nice easy, fuel conserving cruise of 25 kts at around 3500 +-. You may have some other problems going on. However, I'd bet that if you went to a 15 x 17 prop, you'd notice better all around performance out of your MT.

You may want to check your installed boat tach against a good dwell-tach. Put 'er on the water and run WOT. Work with a buddy to compare RPM at the engine to the instrument panel tach. I thought my RPM was down some, too. But when I did this on-the-water comparison, I found my 1987 OEM tach showed as much as 800 RPM below actual RPM at WOT. I replaced the tach & get accurate readings now.
Cheers!
JSMoore

 

[john cole]

Re: Dwell setting

yes you did reply in another forum, i pm'ed you asking you what carb you have on your engine too? i checked my tack and at a idle or fast idle is is off it was up to a couple of hundred off, so i'm wondering that as i increase rpms while under way if the gap of errror also increases. i didn't have time to check it or a assistant with me to watch it. but i confident i have taken care of the electrical end of teh problems and i believe the fuel end too (however i'm still going to swap out the fuel water sep again). so i'm most likley will see if i can find a 15 X 17p prop to try. i have one guy here who will loan me a 15p prop to try but i feel that will be under pitched for what i need.

thanks

 

[sethjon]

Re: Dwell setting

You can down size the prop but you won't get much more out of it. It sounds just fine to me and the revs are kept down a little which is easier on the motor. I wouldn't change a thing. My sons boat has the same engine but I think he has the 4 bbl. I might be wrong and his prop is 21. He used to use the 10 and while the hole shot was a little better its barely any different.

 

[bruceb58]

Re: Dwell setting

You can down size the prop but you won't get much more out of it. It sounds just fine to me and the revs are kept down a little which is easier on the motor.

Not if he is lugging his engine...BAD ADVICE!

 

[john cole]

Re: Dwell setting

well i did alittle more checking and followed some advice and advanced the timing a bit (didn't have my timing light and don't have one with the advance feature, going to borrow one) and i picked up the rpms i'm missing i got up to 4100rpms at 31 mph. big improvement, i didn't push it any harder because on not having the light and not wanting to wreck anything. so while the timing seemed ok on the lower end it must not be advancing all the way for this motor. so i looks like i have this gremlin in the crosshairs finally. so if someones for sure what the max advance is for this motor i would apprecitate the info. once again it is a 1987 4.3l omc with a 14.5 X 19 aluminum prop pushing a 1987 mark twain.

thanks

 

[bruceb58]

Re: Dwell setting

Agree with don't doing anymore running of your engine without correctly setting the timing. You did set the timing at idle with a timing light right? You used a dwell meter right?

 

[john cole]

Re: Dwell setting

yep set the points by the dwell reading, set the idle to but i did mine at 6 degrees at about 900rpm and didn't have the right timing light to go any higher (no advance on light), will be borrowing one with it to confirm dist advance. and following advice in another thread i'm working, is i will be pulling the dist again and giving it a good cleaning to be sure there is nothing to hang up the weights, even thou they seem to be moving ok now.

 

[bruceb58]

Re: Dwell setting

You can also mark your balancer if you want or use timing tape although less precise than having the light. Having an advance light is nice even for setting your base timing.

 

[john cole]

Re: Dwell setting

how would you use the tape, any tips on who would have it?

 

[bruceb58]

Re: Dwell setting

Depends on the diameter of the harmonic balancer:
http://www.summitracing.com/search/...lancers/Part-Type/Timing-Tape/Make/CHEVROLET/

A local auto parts store may carry it.

 

[jeromeb]

Re: Dwell setting

Agree with don't doing anymore running of your engine without correctly setting the timing. You did set the timing at idle with a timing light right? You used a dwell meter right?

Take into consideration that distributor degrees are half of crankshaft degrees. 18 dist degrees read 36 crank degrees.

 

[bruceb58]

Re: Dwell setting

Take into consideration that distributor degrees are half of crankshaft degrees. 18 dist degrees read 36 crank degrees.

The advance chart listed is for the timing measured at the crankshaft. He wants to be measuring 18? at the crankshaft.

If you have timed your engine assuming this, I suggest you retime your engine.