Breaking up under load...

[techspecial]

Finally splashed my Minx...Went from the stock 260 to something around
380HP from Summit Racing...One of the Blueprint Eng. 255c.i. engines. I stuck with the stock T'bolt IV ignition. After this thing battling with me for about three months I finally took her out and it breaks up very badly when I increase throttle. To the point where the tach. jumps around. I still have the old Q'jet that's set up for the 260. Am I starving the beast? I imagine it's a little bit thirstier than the old engine. I haven't touched the carb at all, ran out of daylight. Bad gas? Any thoughts would be greatly appreciated!!!
Thanks, Mike:confused:

[Donziweasel]

What is the CFM of the carb? The engine is a 355, not a 255, right?

[harbormaster]

You are going to fry that engine with a lean condition if you don't get some carb on it.

[techspecial]

355 not 255...sorry. Don't know the numbers for the Q'jet.
I only goosed it a couple of times...never above 3000.
Carburation is the culprit??? Remember the Q'jet is still set
up for the 260. Is there an adjustment on the Q'jet that will
open it up of does it require a jet replacement?

[harbormaster]

Mike I know there are alot of Quadrajet fans out there, but I would put a Holley on that bad boy. Qjets are not bad BUT more people know how to work on them than Rot chesters

[techspecial]

Yeah...That's the direction I'm leaning. Not many fans of the Quadrajunk or was it Quadratrash...I guess I'll call Blueprint on Monday and see what they recommend...Any recommendations from the board? I have a buddy who's lending me one tomorrow for a sea trial...

[Donziweasel]

600-650 cfm should be fine..... My 355 (377 HP) had a 600 cfm. :wink:

[BUIZILLA]

what intake manifold?

[MOP]

Well everyone is leaning toward carb, the tach jumping has me thinking electrical!
Tbolt is a good setup, I would look at the coil and the distributors pickup. Also make sure the Tbolt pack has a good ground at the base of the distributor, I have made the mistake of not tightening that particular ground which drove me nuts for two days. My guess is one of the easiest to check, if you have an outboard shop near by I bet one of the techs has a set of inline sparkies. They have saved me a ton of time over the years determining fuel/electric issues, if you can borrow a set they will tell you one way or another.

[techspecial]

Hi Phil, The very ground wire that your talking about was in very bad shape. I really thought I was on to something. I chopped it and re-lugged it and it wasn't the problem. I've also replaced the cap and rotor, checked firing order,did not check plug wire resistance yet. The pickup in the distributor looks undamaged but you know how fragile those little buggers are. What are sparkies. I'm thinking that they're do-hickeys that you put between the plug wire and the plug and they flash. Am I on the right track? When it's idleing the tach. is steady it only starts to fluctuate when it starts breaking up. Doesn't look like I'm going to try the test carb today. Weather is keeping my carb lender all cooped up today...Thanks for all the input..
The intake was supposed to be a Brezinski or something like that but it looks like a dual plane Mercruiser (it has the mercruiser name cast in to it)...

[techspecial]

And another thing...I'm running September gas. I did add stabilizer and Startron in it before winter though...

[HOWARD O]

Got a good filter? ie fuel/water seperator?

[VetteLT193]

I'm with MOP. Electrical. What's the volts reading? you might have a short somewhere, or something is not connected properly.

too little carb really shouldn't show up until past 3/4 throttle

[BUIZILLA]

i've spent thousands of dollars with Brezinzski...

not for a boat though... :wink:

[BUIZILLA]

no big deal..... just move the exhaust ports... :bonk:

[Ghost]

Finally splashed my Minx...Went from the stock 260 to something around
380HP from Summit Racing...One of the Blueprint Eng. 255c.i. engines. I stuck with the stock T'bolt IV ignition. After this thing battling with me for about three months I finally took her out and it breaks up very badly when I increase throttle. To the point where the tach. jumps around. I still have the old Q'jet that's set up for the 260. Am I starving the beast? I imagine it's a little bit thirstier than the old engine. I haven't touched the carb at all, ran out of daylight. Bad gas? Any thoughts would be greatly appreciated!!!
Thanks, Mike:confused:

Would it help folks here to know what RPM ranges correspond to the symptoms above? (Is it FINE up to some RPM, and then starts breaking up above that? I got a little lost trying to understand the symptoms.)

Mike

[mrfixxall]

I just had the same problem with a baja,tach was jumping and engine was cutting out over waves!! we replaced the ignition coil and it runs like a raped ape now :)

[Jraysray]

I just had the same problem with a baja,tach was jumping and engine was cutting out over waves!! we replaced the ignition coil and it runs like a raped ape now :)

Simple lose ignition wire on the back of my dash fixed my problem. Had the same symptoms.

[MOP]

Hi Phil, The very ground wire that your talking about was in very bad shape. I really thought I was on to something. I chopped it and re-lugged it and it wasn't the problem. I've also replaced the cap and rotor, checked firing order,did not check plug wire resistance yet. The pickup in the distributor looks undamaged but you know how fragile those little buggers are. What are sparkies. I'm thinking that they're do-hickeys that you put between the plug wire and the plug and they flash. Am I on the right track? When it's idleing the tach. is steady it only starts to fluctuate when it starts breaking up. Doesn't look like I'm going to try the test carb today. Weather is keeping my carb lender all cooped up today...Thanks for all the input..
The intake was supposed to be a Brezinski or something like that but it looks like a dual plane Mercruiser (it has the mercruiser name cast in to it)...

I have are two types of sparkies, neon inline and gap jump. The neon's will let you know the spark circuit is completing, the gap will show you the intensity. They don't compare to a scope but then again a scope is hard to put in your pocket! I feel marine engines due to the constant load carried require a healthy spark, higher compression seems to need even a bit more. You are not running your old spark plug wires are you?

[seano]

Check the coil. A good indicator is if it is actually warm/hot to the touch...

[techspecial]

New wires tomorrow...And a Holley double pumper...
New coil today...And a dedicated ground for the ignition module.
(Didn't know if it was getting a good ground through the Stainless
Marine fancy shiny riser)..Same symptoms...Next I'm going to jump into the dash wiring. Something just ain't right back there with the tach. jumping around...THANKS GUYS!!! Mike

[mrfixxall]

New wires tomorrow...And a Holley double pumper...
New coil today...And a dedicated ground for the ignition module.
(Didn't know if it was getting a good ground through the Stainless
Marine fancy shiny riser)..Same symptoms...Next I'm going to jump into the dash wiring. Something just ain't right back there with the tach. jumping around...THANKS GUYS!!! Mike


mike did you chect the pick up in the distributor,,thats the one i was refering to..

[techspecial]

No I haven't. How do you do that? I've had to replace them before but they failed completely. The distributor was on my bench getting moved around a lot for a long time. I know that they are really shock sensitive. I've destroyed them before just by removing a stubborn cap. I'm open to all suggestions though!!! I'm an instrument guy at a nuclear power station...This seems an awful lot like work!!! Intermittent problems drive me crazy every day. Bring on the Homer Simpson jokes...

[MOP]

Life can be simple!!!!! Fuel will in no way effect a tach!!! There for dig into electrical!!!

[mrfixxall]

No I haven't. How do you do that? I've had to replace them before but they failed completely. The distributor was on my bench getting moved around a lot for a long time. I know that they are really shock sensitive. I've destroyed them before just by removing a stubborn cap. I'm open to all suggestions though!!! I'm an instrument guy at a nuclear power station...This seems an awful lot like work!!! Intermittent problems drive me crazy every day. Bring on the Homer Simpson jokes...

you need a dvom to ohme them out or a power graphing meter and ck for spikes on the graghs.! also look for rust, the older ones rusted and threw it around under cap and made ir misfire.the newer pc coils are totally sealed.

[techspecial]

It was ignition...The module that I originally had for the 260HP didn't have the right advance factor for the new beast. At low RPM's it was just fine then it started to go out of kilter at 2500 and got progressively worse as it approached WOT. Turns out the model number of the module includes how much the total advance will increase to max. A plug has to go out to Joe Jones at Barber's Basin in Salem NJ he nailed it right away!!! I'm telling you it just flies now...Cruise speed is now close to my old WOT...You can't slap the grin off of my puss right now!!! THANKS FOR ALL THE HELP GUYS!!! I still may go with the Holley eventually, but the plugs look real good now.:):):):):):):):):):):):):):):):):):):):):):):) :):):):):):):):):):):):):):):):):):