75 Mark Twain resto, low rpm bog/backfire

[IWELD]

Thanks to the wisdom of fellow iboaters I'm almost complete with my restore. Currently I'm having engine issues and I'm stumped!!

1975 Mark twain V-Sonic, 21 ft
Mercruiser 888 drive (ford 302) with pre-alpha drive
~added~ Carb: New Edelbrock 4bbl 600cfm (1409)
Intake: Edelbrock performer 289

Alt and starter were rebuilt by professional.

new: fuel pump, points, condenser, cap, rotor, ignition coil, plug wires, plugs

This is my problem, engine starts great, idles and revs in neutral on water and muffs great....sounds awesome, idles in gear on the water just fine but when I accelerate and get to about 1200 RPMs it bogs down and backfires thru the carb and I can't accelerate thru it. I have to return to neutral else engine dies.

What I've Done:
Cleaned distributor mechanical advance weights, seems in good order,
replaced points, rotor, condenser, Ignition coil (external resistance), timing set at 10? BTC. ( have tested other settings thru 30? but still same issue, thought markings may be off).
Replaced fuel water separator, Checked fuel pressure at carb (4psi), no air bubbles visible using temporary clear fuel line while doing pressure check.
Carburetor was new but I removed it, dissembled and soaked it just in case reassembled and reinstalled, no change in performance. Also checked for vacuum leaks around carb and found none.
Replaced plugs, gaped to .030" and checked compression,

Compression check results:
Cyl
1, 100psi
2, 120psi
3, 105psi
4, 100psi
5, 100psi
6, 100psi
7, 110psi
8, 110psi

I have double and triple checked plug wire routing and firing order but i have not physically checked No. 1 TDC and compared to distributor yet.
Edit: No. 1 verified, firing order correct.

Not sure where to go next, stuck valve? or bent rod maybe?

I do have mercruiser service manuals and use them.


Any insight would be great, i'm not throwing in the towel yet but I hate to have to take it to someone else and spend cash on things i know i can handle.

Thanks,
Iweld

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

Could blended fuel do this? I'm thinking I should find some 93 Min octane gas that's not blended with ethanol. I didn't consider this till now.

Thoughts?

 

[carcraze]

Re: 75 Mark Twain resto, low rpm bog/backfire

Well doubt it is a stuck valve as the compression test would have shown that, I also doubt it is blended fuel but I do not have too much experiance with the blended fuel thing. Most of what I have read deals with ethanol pluging things up due to incompatible fuel line or fibreglass fuel tanks. Could be a vlave not opening because of cam lobe or bent push rod. Could check that with removing the valve covers and checking valve lift and clearance.

Did it hold fuel PSI at the throttle position when it bogs? When the tune up was done did you dwell the points?

Just throwing some things out there.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

Well doubt it is a stuck valve as the compression test would have shown that, I also doubt it is blended fuel but I do not have too much experiance with the blended fuel thing. Most of what I have read deals with ethanol pluging things up due to incompatible fuel line or fibreglass fuel tanks. Could be a vlave not opening because of cam lobe or bent push rod. Could check that with removing the valve covers and checking valve lift and clearance.

Did it hold fuel PSI at the throttle position when it bogs? When the tune up was done did you dwell the points?

Just throwing some things out there.

Thanks for the hint on checking lift and clearance, I'll check that tomorrow.
The dwell is set at 29? and I didn't check the pressure at the ramp, I just checked it on muffs. I can double check it in the water if it's possible for the pressure to drop in gear.

 

[haulnazz15]

Re: 75 Mark Twain resto, low rpm bog/backfire

Not that I could imagine this would make a difference, but I believe you should be setting it for 8 degrees BTDC. That being said, I'm positive our old 351w was set at 10 by the PO and it ran great. I can't imagine 4psi being bad, but I know you should go over 6psi for a carb. Have you checked to make sure your vacuum is good at the carb (no vacuum leaks) since you just replaced the manifold/carb?

If you see my sig, I have a '76 MT with the 351w, so I should be able to help you with any pictures you might need. I even have the engine on a stand at the moment waiting for 1 freakin part to come in before I get the boat back in the water after 2-years (also upraded to the 1409/edelbrock perf. manifold).

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

I'm pretty sure the service manual calls for 10? BTC but I'll double check. And no I have not measured the vacuum at the carb yet, It seems i hid my good pump/gauge
I tried using my combo gauge but it didn't register any vacuum at all at idle so I assume it's not working, I'm also going to recheck the fuel pressure when I find it. I was thinking that 6psi from the pump should be max and 4psi was okay. I'll have to research edelbrock and check.

Thanks,
Iweld

 

[haulnazz15]

Re: 75 Mark Twain resto, low rpm bog/backfire

You're correct on the 10-degree base timing. I was just recalling the wrong number. Have you ensured with a timing light while moving that the distributor is advancing the timing? If the engine is bogging under load, the distributor may not be advancing timing properly, which you could only check while in the water.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

How do you know how much advance it should have at a givin rpm?

 

[haulnazz15]

Re: 75 Mark Twain resto, low rpm bog/backfire

I'm sure there are charts you can use to verify, but I just meant for you to use the timing light while on the water to verify if was advancing the timing under load. The only real concern is making sure it doesn't advance past the maximim allowable (36 total degrees I think, don't quote me).

 

[sully73]

Re: 75 Mark Twain resto, low rpm bog/backfire

thats the first thing that i thought of was the advance. malybe the springs in the distib are bad or not functioning right. i wouldnt think youd get backfires from low fuel pressure, maybe just bogging. sounds alot like a timing issue with the backfires.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

I havn't had a chance to check the advance under load yet but I will. It seems like its a timing issue to me too but I just hav'nt got it nailed down yet. I have removed the advance weights and cam to check and cleaned them up a bit but they seemed ok, no dried grease or rust to bind them up, but the light weight spring is a bit rusty but it has good tension, the heavier spring stiil looks new /no rust.

I'll try to google an Ignition Advance Curve graph to see if i'm in the ballpark. maby the rusty spring is not origanal and is the problem.

Or Maybe DON S will search thu his archive of wisdom if he happens to read this.....

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

Have you ever had one of those Ah-Ha moments where at first your astounded by your brilliance and then moments later your disgusted with your self? Mine was today.......

While driving to lunch from work I was playing back in my head all the things I had done to troubleshoot the backfire issue, then suddenly I realized, thinking of all the times I've connected the leads of my engine analyzer (to get RPM readings) that there was only one wire on the Neg side of the coil........The tach sending wire was missing I checked when I got home from work and sure enough the tach lead was on the positive side. Quickly switched terminals and Whammo....Tach worksDisgusted.

Now, I haven't had it the water yet but it makes since to me that I had the coil virtually grounded thru the tach and hence getting SHi**ty spark and low power in gear.

When it stops raining I'll get it in the water and test it out...

 

[[OBC]Patch]

Re: 75 Mark Twain resto, low rpm bog/backfire

I'm sure there are charts you can use to verify, but I just meant for you to use the timing light while on the water to verify if was advancing the timing under load. The only real concern is making sure it doesn't advance past the maximim allowable (36 total degrees I think, don't quote me).

You dont have to put it in the water to check distributor advance, its a rpm related thing, not vacuum(load), like autos. Just run it up and see if line moves out further. I's sure its just the points, anyway.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

I did check the advance after i switched the teach lead on the coil, It advances fine, from 10? at idle to 30? at about 3000 rpm.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

I have finally found the problem, the tach had nothing to do with it. It turned out to be the firing order. I can't explain it but the firing order in the service manual specs 1 3 7 2 6 5 4 8 is what I had, I took a buddy with me who lives next to the lake I test the boat in and he was stumped as well, long story short we re back at his garage and he digs out an old 1972 ford engine book and we check the firing order in it and the same firing order in the book was for a 351! so just for the heck of it we used the firing order for a 72' 302 which is 1 5 4 2 6 3 7 8, turned the key and it ran so we hauled *** back to the ramp and no more backfire! I guess hitting on four cylinders was enough to run in neutral but i figure it would backfire more.

Now for the question I don't want to ask, its obvious now that this is not the original engine for a mercruiser 888, So what the hell is it from? Will it be OK? I assume it was for the previous owner but it sat for 20 years before I bought it. It had all the correct marine parts, starter, alt, intake, carb and such. If it has the wrong cam is this going to be an issue?

 

[haulnazz15]

Re: 75 Mark Twain resto, low rpm bog/backfire

To my knowledge the 302 did NOT have the 351w firing order until the 302H.O. in the 83-95 Mustangs, etc. Your boat should have the pre-83 (HO) firing order which starts with something like 1-5-4-2-6-3-7-8. If the cam was swapped to a 302HO cam or 351W cam, it would use the 1-3-etc firing order.

The marine engines may be different, but that info should be correct for Ford 302's in general.

 

[IWELD]

Re: 75 Mark Twain resto, low rpm bog/backfire

Finally got out on some open water for a few hours today, Fist time for this boat in 21 years. It ran great, so I'm glad to be past the backfiring issue. It never missed a beat. I was able to GPS the speed, 37 MPH @ 4500 RPM (by the original tach gauge). The RPM's are a bit over the mercruiser specs at WOT if I remember i think it should be 3600-4200. I've got a 19P prop which did slip once out of the hole so I'll take it in and get a new bushing pressed in.


all in all a great ride.....thanks for checking in and lending a hand

Iweld

 

[[OBC]Patch]

Re: 75 Mark Twain resto, low rpm bog/backfire

The changes stemming from the, "wrong cam", would be; failure to start if it bent valves on the first cranking, poorer performance if the cams peak power rpm range is other than what is typically run in your boat. If it runs, you're probably OK! Happy wakes