help with Q-jet stumble and maybe down on HP (6-20-08 up date)

[nailem]

first off thanks for all the help on my shift issue see: cant shift out of gear, please help (update 5/28) thread.

i have a 86' 20' sunrunner 220cv with a 5.0l 230hp and alpha 1 drive. when i hammer down from idle it kind of coughs then goes. not really a back fire but a cough. not ever time but most of the time. timing was set at idle to 9* BTDC. i know it should be 8* but i got 9 and thought it would be close enough. should i advance it some? what else could be causing my stumble?

over at the prop forum they seem to think im not putting out the full ponies. i have a 14.5 x 17 prop on and can only get 4000 rpm lightly loaded. they feel i should be able to run a 19 pitch

i put new plugs, cap, rotor, and oil change. carb was disassembled and cleaned. accelerator pump was in good shape and giving a good squirt of fuel. old fuel drained, check filters and replaced water seperator.

things i guess i need to check:
compression, fuel psi...... what else??????

i did use 1qt of 80w140 gear lube and 1/2qt 80w90 when i did the out drive. would that rob that much HP? i do have 2qt of amsoil 75w90 i could throw in it, is that ok to run? thanks

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

it is a 4bbl with a single pumper. when i took the carb aprt to clean the pump looked to be in good conditon. i adjusted it to the oem spec of 23/64 on it hight.

im not sure what the idle cutoff solenoid is. where is it located? its raining again here but i will check the hose going to the secondarys. what other hoses are there? i didnt even think it had a PVC valve but then agian i wasnt looking for it. thanks and i'll let you know how i make out.

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

Here's a few Qjet hints, maks sure you didn't lose the check ball under the accelerator pump, it should shoot a healthy dose of fuel in when opening the throttle, next, under the air horn (cover) in the middle of the primary barrels is the spring loaded power valve, it is held in place by a removable snug fit plastic grommet, and moves the primary metering rods up, & down, controlled by engine vacuum, and has a spring underneath it to govern it. Be sure that it moves up, & down, smoothly against the spring pressure (these are notorious for sticking) you will have no power if this valve is gummed up, or stuck, a healthy dose of carb cleaner maybe needed. Aside from removing all the jets that are removable, and cleaning out all orifices good, that's about it for the Qjet.

By the way, a boat carb has no PVC, EGR, nor heat tube, just probably an electric choke.

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

thanks again mikdee, i didnt think i saw a PVC. the only hose i saw was from the vacuum diaphragm to the carb, is there any other i should check? also when i had the top of the carb off and the accelerator pump out i didnt see a check ball. there was a spring, the pump rod. on the rod there was a blue rubber "umbrella" with little springs. does the check ball sit in the bottom of where the pump rod sits? i cleaned all the jets and the float did not seem to stick. i believe the the spring that controls the the metering rods was working freely.

i was reading in another post about two different methods of checking you timing base on witch version of thunderbolt you have. how do i know what i have. i just plugged the timing gun into #1 cylinder and timed. the other method talks about jumpering wire 13. not sure were that is. i guess i really need to find a compression tester and get that done.

i'll probable just respond to this thread now unless more jump in on prop forum.

 

[John_S]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

If stock, not T5 which came out around '95-'96. Not sure when T4 started, but do you have an external ignition module or points?

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

thanks again mikdee, i didnt think i saw a PVC. the only hose i saw was from the vacuum diaphragm to the carb, is there any other i should check? also when i had the top of the carb off and the accelerator pump out i didnt see a check ball. there was a spring, the pump rod. on the rod there was a blue rubber "umbrella" with little springs. does the check ball sit in the bottom of where the pump rod sits? i cleaned all the jets and the float did not seem to stick. i believe the the spring that controls the the metering rods was working freely.

i was reading in another post about two different methods of checking you timing base on witch version of thunderbolt you have. how do i know what i have. i just plugged the timing gun into #1 cylinder and timed. the other method talks about jumpering wire 13. not sure were that is. i guess i really need to find a compression tester and get that done.

i'll probable just respond to this thread now unless more jump in on prop forum.

The check ball rolls around in a little passageway under the accelerator pump plunger. When you press down on the plunger, it blocks off a fuel passage, forcing fuel out of the pump jets into the carb, when pressure is released it rolls back, & opens another passage for fuel flow thru the carb body. If the carb is upended during rebuild, the ball can drop out, & get lost way too easy
ask me how I know? lol

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

it has an external module that is mounted on the exhaust manifold. going by memory it is about 4" square. by maybe 1" thick. my buddy that has the same year his module is much bigger and looks older than mine.

mikdee, i would say i did not lose the ball, i get a good healthy squirt in each bowel when pushed. any comment on watching the secondary open? under load or just rev it.

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

The ball is probably still in there, or it would be alot worse running. Now, Never free rev your engine if you can help it I've seen bad things happen! Or, you're left with a "smoke machine" (oil burner), been there done that, many yrs ago, put a hollywood deep tone glasspack on my old pristine 56' Olds 88, kept revving it to hear the sweet sound, eventually it was followed by blue smoke! making more each time, can you say worn out rings?

Did you set it up with the engine off first, like I said? You might have to lift the rear butterfies to look down the carb throat? Next, When on the water, take off your flame arrestor (temporarily) make sure it's under load, then see if it's opening all the way.

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

nailem, You did clean, or replace, that small cylindrical plastic fuel filter behind the big nut on your carb where the fuel line goes in, Right?
And the spin on external fuel filter/water seperator? I always like to check out my antisiphon valve, & in tank filter as well, ever since I found out every boat has one! unless it's been removed, or tinkered with I know sometimes this is a hassle but a necessary evil

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

i did replace the water separator and check the filter in the carb (clean as new) but not the one in the tank. i also did not check the ant siphon. i will next trip to the boat.

i figured it was not good to dry rev the motor. i did some sport bike racing a few years back and was told this at the track. good possibility to float the valves. you always here these young kids revving there bikes.

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

just wanted to up date 6/8/08. it is for sure down on power. i went out with my mom, nephew, wife and kids maybe 300lbs more than the last time and it could hardly get on plain again. took a long time and still no revs over 4000rpm. i put new wires on it before we when out . i still need to check compression and the secondary. im leaning toward the secondary or i should say hoping.

i did check the anti-siphon and in tank filter, both were fine.

 

[Maclin]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

Check throttle linkage to make sure it has full travel.

And definitely check the secondary air valve spring tension, it may be too tight and not opening, or even too loose and opening way too soon and too much and the engine cannot ever "catch up".

Though if it is opening too much there should be a horrible bogging sound as the secondary airvalve falls open, and the engine would feel like it is lugging down then trying to rev back up again.

 

[rodbolt]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

the accelerator pump check ball on the Q-jet is located under a screw between the pump bore and the needle valve bore.
your not really describing a carb isue.
the q-jet, like most all vacum seconday carbs will give a slight hesitation,(uggggg,,,,,,, waaaaaaaaahhhhh,) when nailed from a low speed condition. its simply how they work.
you really need to figgue out which ign system you have so you can insure its properly timed, unless monkyed with the secondry air valve tension wont change.
if your going to play with a Q-jet you will need to read up on how and why the 6 circuits in that carb are there and what each does.
the marine version is usually the simplest due to lack of emission control stuff such as anti-diesel solinoids,float chamber level solinoids and hot idle compensators.
simply did not need them.
its probably the most widely used 4bbl carb in production and works extremely well due to the triple venturi primary setup and vacum operated secondaries.
the basic design has been in production since 1965 with a few mods for chokes and emission controls.

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

Yeah, I wouldn't mess with the air valve spring adjustment, unless you're a carb expert. With all the info I gave you, you never mentioned if you checked out your throttle linage to see if the 4bbl is opening, at least with the controls and engine off? Anyway, if it's not that, I'd suspect timing issues, as long as your compression is good?

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

thanks for the links, i'll print and read.
i check the secondary yesterday and the definitely open all the way. also it seems like the throttle linkage travels until it his the stop on the carb. i have a thunderbolt IV ignition so i shouldn?t have to jumper any white wires or ground a wire correct? if not my initial timing was set to what i would call 8 1/2 BTDS and i adjusted to about 9 1/2 and the stumble seems better. if you punch it it is better than before. i can give it a nice controlled throttle increase and it does not do it at all. so i think i will just adjust my driving habits.

as far as it being down on power. i guess it probably still is but it sounds good when given a rev in neutral and it does not blow any smoke or burn a considerable amount of oil. i have replaced everything in the ignition except for the coil. and checked the anti-siphon, and both fuel filters. ( i think its getting plenty of fuel) the only thing i have not checked is the compression and i will probably get flamed for saying this but i think im going to wait until fall to do so. even if the compression is low i am not going to pull the motor right now and rebuild it right now. i bought the boat for family time with my to small girls and dont want to spend half of summer with a motor out.

unless you guys have any other suggestions im probably going to try buying the 14 3/4 X 15 4 blade prop off of this site and see if it raises my rpms. or if you really think i should still worry about it have it looked at by a professional. i really appreciate all of your help and time.

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

i got my new 14.75 x 15 4 blade prop from iboats. fast shipping by the way. i went to put it on yesterday while anchored on the beach. probably not the best idea but i was careful. the old prop is a 14.5 x 17. slid new prop on and it does not clear my brand new anode. i know they sell flat anodes but i just bought this one. the prop seems to just barely hit the anode. my question is can i grind or file the anode just a little to gain the clearance i need with out any adverse effects? and is there a minimum clearance i should keep between prop and anode? do i need to allow deflection of the blades?

since i advanced my timing to 9.5* my stumble is all but gone. and it also seems to start easier

 

[Bondo]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

if not my initial timing was set to what i would call 8 1/2 BTDS and i adjusted to about 9 1/2 and the stumble seems better.

Ayuh,....
Put the Timing back where it Belongs,...
You're running the risk of burnng down your motor the way you're going....
Fixing a Fuel Issue with Timing will get you an Anchor instead of a Motor....

Btw,..... Those old Q-Jets are Known to get Loose throttle plate shafts, that'll cause a Lean condition from excess Air leaking into the air flow.....

do i need to allow deflection of the blades?

Ayuh,.... Grind it off til you have 1/4" clearance....

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

thanks bond-o, i'll set the timing back. just seems to start better and with no stumble. were are the throttle plates shafts located? i have the break down on the boat but im not sure of what you are talking about. i have tried everything else i can think of.

 

[nailem]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

ok i installed the new prop and my rpms are up to 4400. comes out of the water nice but i cant get over i should have more power. so i was reading Don S instructions on how to test your fuel pump and i dont fully understand how to do it. below is a picture of a vacuum & fuel pump tester. will this work? i have read the instructions over and over and im not sure why i dont get it.


also i was reading another thread about the outdrive possibly being changed at some point with the wrong ratio. below is also a picture of my drive and i cant find where its stamped. i see "B344554" and a large "F" thats it. am i looking in the wrong place?

 

[MikDee]

Re: help with Q-jet stumble and maybe down on HP

Re: help with Q-jet stumble and maybe down on HP

Yeah, I wouldn't mess with the air valve spring adjustment, unless you're a carb expert. With all the info I gave you, you never mentioned if you checked out your throttle linage to see if the 4bbl is opening, at least with the controls and engine off? Anyway, if it's not that, I'd suspect timing issues, as long as your compression is good?

What I meant by timing issues was, maybe the timing mark on your vibration damper (crankshaft weight, & pulley) has slipped, moved, and you're running abit retarded (re: low power, hesitation when powered up) The only way to check this is pull out #1 plug, then bring the crankshaft around manually with a wrench, until you can determine the piston it is at the top of it's stroke, and see where your crankshaft pulley mark is? (if they line up, or not?)