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Kmfish87

Engine Boggs at High rpm

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Kmfish87

I have a 1995 response with the 350 tbi engine and ~620 hours on it. the engine cranks up and runs nice and smooth every time. The only issue I am having is  that I cannot get the engine to rev past 4000rpm while under load. It pulls strong to 4000 and then slows down the more throttle I give it until I back out of the throttle.

I replaced the spark plugs with NGK brf6s, replaced the fuel filter, pulled the vapor separator tank apart and cleaned the fuel pick up screen as well as the vapor tank itself out, and cleaned the flame arrestor. While I had the flame arrestor off I checked the injectors and they both looked to be spraying a very clean cone pattern with no dripping or anything like that. I put the boat in the lake this weekend, got it up to operating temp (160) and then attempted to check the timing... I hooked in a jumper wire to the service plug jumping pins A and B together to put it in service mode. When I attempted to start the engine it did not want to start until I raised the throttled up some. The idle was elevated so I backed the throttle back down to the neutral position where it struggled to run for about 20 seconds and finally shut off on its own. I raised the throttle again and started the engine, set it to around 1000rpm via the tach (as this is what I read base timing mode should be automatically) and checked the timing. it was jumping between 8 and 12* btdc. I said screw it, pulled the jumper and timing light out and ran the boat like it was where I wouldn't ruin our weekend and just kept it below 4k rpm. I haven't had a chance to look at it since saturday morning. I am going to pull the distributor cap today and check it and the rotor. I'm assuming something is going on in the distributor or coil to cause this issue and would be the root cause of my bogging on the top end. Has anyone had a similar issue or have any suggestions on what else could cause this? The dist cap looks to have been replaced recently and the coil looks original to the boat. I have only had the boat since the beginning of the summer and I bought it with this issue.

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Woodski

@Kmfish87:  on the fuel side if the anti siphon valve is sticking shut that can cause a vacuum condition in the fuel tank with symptoms like you see.  Timing should be more solid with less than +/-1 degree of variation, that sounds like worn distributor gear, you can check that at the rotor.

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Falko

Couple of thoughts,

1. HAve you checked the fuel pressure? To me it almost sounds like you have too much pressure and running rich at higher revs. DO you get any fuel smell?

2. How quickly does it fall off at 4k? Could you be overpropped?

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Kmfish87
9 hours ago, Woodski said:

@Kmfish87:  on the fuel side if the anti siphon valve is sticking shut that can cause a vacuum condition in the fuel tank with symptoms like you see.  Timing should be more solid with less than +/-1 degree of variation, that sounds like worn distributor gear, you can check that at the rotor.

I will check the dist tonight after work. Had to cut the grass last night. I have opened the fuel fill cap while running to see if that makes a change but it didnt seem to have any effect. I will pull and clean the anti-siphon valve anyways.

 

55 minutes ago, Falko said:

Couple of thoughts,

1. HAve you checked the fuel pressure? To me it almost sounds like you have too much pressure and running rich at higher revs. DO you get any fuel smell?

2. How quickly does it fall off at 4k? Could you be overpropped?

Any advice on how to check fuel pressure on TBI engine?

It drops off until I pull the throttle back from full to around 3/4. Stock 13x13 prop in good shape, no big nicks, dings or cracks.

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Falko

I don't know enough about the TBI system to know if there is a "secondary" jet for higher rpms like a carb. I doubt there is. Does it taper off as you approach 4k or just fall flat on its face? There might be an issue with the knock sensor messing with the timing as well.

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Kmfish87

Played hookie today and went to the lake...

changed the fuel filter and rotor button this morning and filled her up with fresh 90 non-ethanol. Rotor button tip was pretty worn but distributor cap looked new. Dist shaft was tight.

engine will free-rev to 4800rpm in neutral.

boat still bogs past 3800rpm. Forgot the timing light to check timing again. Found there is a wire that needs to be unplugged (tan wire coming from ign module on distributor) to check base timing. Will do that this weekend. I've read on some sbc forums that the ign module can cause this problem so I may try one of those. They run about $45 from the parts store

@Woodski is the anti-siphon valve on the fuel vent or supply line on the tank. Both fittings look the same coming out of the top of the tank just one goes to the vent and one goes to the fuel filter...

@Falko I don't think the tbi has any kind of secondary injection.

does anyone have a 13x13 prop I can try to make sure mine isn't the cause? Maybe it's been re-worked or bent in some way that I can't see. Or any suggestions on prop-shops around Atlanta to have it checked? Mine appears to be the original and has 13x13 stamped in the hub.

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Kmfish87
2 minutes ago, MadMan said:

Monitor the fuel pressure while the problem is occurring, this will test everything in the fuel system south of the injectors (pump, regulator, vent, pickup, filter etc.).

 

http://www.harborfreight.com/fuel-injection-pump-tester-62623.html

 

 

"Fits most systems (except for Bosch, CIS-Jetronic and GM throttle body systems)"

not trying to be a smart a** but do you have another suggestion? Almost every kit I've seen says the same thing unless it's several hundred $$

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MadMan
Just now, Kmfish87 said:

 

"Fits most systems (except for Bosch, CIS-Jetronic and GM throttle body systems)"

not trying to be a smart a** but do you have another suggestion? Almost every kit I've seen says the same thing unless it's several hundred $$

Maybe Advance Auto or Auto zone has a loaner?

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Kmfish87
37 minutes ago, MadMan said:

Maybe Advance Auto or Auto zone has a loaner?

Good idea, I'll check after work tmrw. Need to get a few other parts anyways

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powbmps
8 hours ago, Kmfish87 said:

@Woodski is the anti-siphon valve on the fuel vent or supply line on the tank. Both fittings look the same coming out of the top of the tank just one goes to the vent and one goes to the fuel filter...

I have been having some similar issues with my 95, but it's the carb motor so the trouble shooting seems to be a bit different.  The anti-siphon valve should be a ball/spring valve in the outlet on top of the fuel tank.  The fuel line should be attached directly to it.

The timing will definitely jump around if you don't have it in base mode.  Do you have a Thunderbolt V on your boat? 

Edited by powbmps

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Woodski

@Kmfish87:  Powbmps answered your question, anti siphon is a spring/ball check valve as part of the 90 degree hose fitting attaching the hose to the fuel tank.  Should be on the vent line, if it sticks shut and the fill cap is a sealed version that can create a vacuum condition in the fuel tank that works against the fuel pump.  An engine's ability to free rev to or past a certain point is not a good indicator of loaded performance as you note.  The momentum of the rotating assembly assists an unloaded engine to rev past a point that it can accomplish under load, it also takes very little fuel to accomplish it.  You would want to be monitoring ignition timing while performing that task to see if there is scatter, intermittent spark, or incorrect advance to see if engine speed results in an ignition issue, very hard to diagnose any fuel related issues on a free rev troubleshoot.

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Kmfish87
11 minutes ago, Woodski said:

@Kmfish87:  Powbmps answered your question, anti siphon is a spring/ball check valve as part of the 90 degree hose fitting attaching the hose to the fuel tank.  Should be on the vent line, if it sticks shut and the fill cap is a sealed version that can create a vacuum condition in the fuel tank that works against the fuel pump.  An engine's ability to free rev to or past a certain point is not a good indicator of loaded performance as you note.  The momentum of the rotating assembly assists an unloaded engine to rev past a point that it can accomplish under load, it also takes very little fuel to accomplish it.  You would want to be monitoring ignition timing while performing that task to see if there is scatter, intermittent spark, or incorrect advance to see if engine speed results in an ignition issue, very hard to diagnose any fuel related issues on a free rev troubleshoot.

Only reason I posted the free-rev engine speed was to confirm that the engine could, in fact, reach 4800rpm like it is supposed to. I am going to tinker with the boat this evening and will pull the fittings out of the tank and clean them.

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Woodski

@Kmfish87:  From your comment on running with cap open, IMO that indicates the anti siphon valve is not the cause (does not mean not sticking but it is not causing an issue as you circumvented and tested it).  Thinking a bit about the way you comment on boat performance from 4000 rpm on up, it gets to that speed, and as you continue to open the throttle the boat slows down.  If the engine is not getting enough fuel, as you continue to open the throttle plate it will create a leaner and leaner fuel/air mixture which should also mean less power and would result in the engine not being able to maintain speed.  Just a thought.  I would do the fuel pressure check as noted by others, good suggestions on the parts store loaners and most do that these days.  Another option to determine if rich or lean would be to add some fuel (spray can of carb cleaner or similar would work) when you start to encounter the problem, the response would tell you whether fuel is the issue and if too lean or rich at that point.  Not necessarily the easiest process as easy access to the inlet when under load at 4k rpm is not that simple a task but you may be getting closer to wanting to remove the engine box for under load troubleshooting trials.

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Steve B.

I'm guessin' it's over propped. If it starts easily, base timing is good. If it rev's cleanly to almost 5000, high rpm advance is probably good too.

The obvious culprit at that point is the prop. Keep us posted.

Steve B.

quick edit: My 95 with the fuel injection rev's to about 4500. Slightly over propped, but awesome on the take-off. I mean, it takes off HARD !

 

Edited by Steve B.

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Kmfish87
25 minutes ago, Steve B. said:

I'm guessin' it's over propped. If it starts easily, base timing is good. If it rev's cleanly to almost 5000, high rpm advance is probably good too.

The obvious culprit at that point is the prop. Keep us posted.

Steve B.

quick edit: My 95 with the fuel injection rev's to about 4500. Slightly over propped, but awesome on the take-off. I mean, it takes off HARD !

 

It has the stock 13x13 prop and I am in Georgia at pretty low elevations...

I am also not the original owner and do not know if the prop has ever been re-worked. I don't particularly want to take a $500 guess

Edited by Kmfish87

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powbmps
2 minutes ago, Kmfish87 said:

It has the stock 13x13 prop and I am in Georgia at pretty low elevations...

I am also not the original owner and do not know if the prop has ever been re-worked. I don't particularly want to take a $500 guess

May be a stupid question, but how do you free-rev it in neutral?  When mine is in neutral, the throttle lever is blocked from advancing fully.

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Kmfish87
23 minutes ago, powbmps said:

May be a stupid question, but how do you free-rev it in neutral?  When mine is in neutral, the throttle lever is blocked from advancing fully.

pull the silver pin out on the throttle plate that disengages the transmission, then use the throttle as usual.

14175_zps4t2u2l9u.jpg

Edited by Kmfish87

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JasonK

Get full rev in reverse if the neutral button is in front.

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vshockey10

did this ever get fixed?

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