Stubborn To Start 1979 9.8

[keithb7]

Affer a powerhead refresh, my 9.8 won't start unless I squirt some raw fuel into the carb intake. Then it'll fire up first pull, sound wonderful for a second or two, then die. It seems fuel starved. I have done a couple of real good cleanings of the carb in my ultrasonic cleaner. I pulled apart everything I could. Everything in the carb looks very clean. I hooked up the fuel inlet hose with the carb bowl off. I pumped the bulb primer. I manipulated the float by hand. I can see the float and inlet needle & valve are working properly. There is fuel in the bowl. The black plastic ventui is in the carb is present and looks good. The engine just doesn't seem to pull fuel through the carb as it should.

To recap what I did to this powerhead:
Disassembled engine & measured pistons and bores
Honed cylinders
New piston rings OEM
New crank seals OEM
New crank bearings
All new reeds, OEM ( I believe reed block can only be installed 1 way)
Cleaned & gapped spark plugs
Can visually see good strong spark at plugs
Cleaned plug wire connections at plug boots and coils
Set static timing to spec
Disassembled and Cleaned carb in ultrasonic cleaner 2x
Current compression measured at 75 psi, rings not broken-in

I've tried multiple variations of choke, throttle and idle screw adjustments with no improvements. I gave up tonight after the knot in my pull starter handle opened up. All the rope sucked inside. Lol. I'll open that up tomorrow and re-set the rope again. Does anyone have any suggestions or ideas what may be going on here? I'm told 75 psi is normal for the comet and lightening powerhead units. I've hit a wall with this problem. Unsure what is causing the symptoms. Thinking I may swap the carb out with the one off my 7.5. It seems to work good. See if the 9.8 will fire up after a carb swap.

Thanks in advance for any suggestions.

 

[keithb7]

The issue appears to be compression. I put oil in the cylinders and raised compression to 110 psi. She’ll run. I’ll see if seating the new piston rings will improve it.

The cylinder taper and out of round was well within spec. Pistons looked great. Stock bore. New rings and honed cylinders. What do you feel is the issue here? Are these fussy engines that need near-perfect, new cylinders and pistons? Maybe seating the rings properly will do it. Working thru that now. Thx.

 

[rolmops]

It is fuel starved for sure. Sounds like a fuel hose connection or a defective fuel pump.

 

[Nordin]

Just a "long shot", have you change the gasket between the carb and the intake manifold?
I have had this kind of issue with a Mer 200 20Hp.
You have to set the gasket in the right way as there is a little hole in the gasket which is for the crank case pulses to drive the fuel pump.
I suppose your engine has the fuel pump integrated with the carb.
I think the 9,8 Hp and also the 7,5Hp has the same kind of carb with the fuel pump mounted at the left side of the carb.

 

[Mc Tool]

Sure does sound like fuel starvation

 

[keithb7]

Thanks for your responses folks. I feel like I solved it. It was indeed low compression. I lightly honed the cylinders and installed new rings. Compression test dry, netted 75 psi.

I pulled the plugs and put some oil into the cyliders. I pulled it over a few times to coat everything. All the micro grooves from honing were filled. Compression test now showed 110 psi.

I put the spark plugs back in and it fired up first pull. This tells me the new rings need to seat and break-in. I need to do this with the engine under a load. I’ll get my boat out on the water this weekend and do that. I fully expect the compression to rise and then rings seal.

 

[keithb7]

I spent 2.5 hours out on the water today. Breaking-in the engine. When I got home I measured 75 psi still. Seems odd to me after a light hone and new rings. I measured bores and also used a dial bore gage. The cylinders measured to be well within specs. I re-used the pistons. There is no published piston specs in my Mercury shop manual. Do new pistons somehow add to raising the compression higher?

 

[Mc Tool]

You did say in your original post that comp for those engines was normal at 75 psi ....which is what you have . A re-ring and hone dont make a new motor but it should make it shine a bit .
How did the engine go on your break in run ? If it goes well , power is ok , idles .......call it a good un and go boating

 

[keithb7]

Someone told me they were low compression. I am doubting that. I have since seen folks on-line measuring 110 psi. The break-in exercise on the lake yesterday, it wouldn’t start. I squirted some raw fuel in the carb. It flashed up then. High idle and up, it seemed to run quite good. Wide open seemed good. It would not idle. I shut it off and fired it up multiple times out on the lake. Just had the keep the rpms up when it ran.

I worked on the engine again today. There was no way it was starting. I discovered no spark. I measured the ignition system parts and found the triigger out of spec. Higher resistance than expected. I replaced it with spare that I had off a parts outboard I acquired. It had a nice strong spark now. I ran out of time. I’ll work on it more this week.

 

[Mc Tool]

Yeah , 75 psi is about 10 less than I would call a minimum. Did you have the throttle open when testing compression?
I dont spose you noticed if there is a decompression port there . Usually either a hole drilled in the cylinder wall about 8-10mm above the ex port to a point in the roof of the ex port about an inch from the cylinder, or it can be a notch in the top of the ex port window .
These provide a bit of compression relief for starting but once the engine is running they dont have enough open time and compression loss is negligible .
After all that it sounds like you have slow run fueling issues , I would double check you dont have a vacuum leak or a low speed jet /transition port blockage.

 

[jimmbo]

You're certain the Float is set correctly?
Don't go drilling Holes in the Piston, if they did any good Mercury would have already done it
Describe your Break-in Process?
What did you use to Home the Cylinders? Please don't tell me a Ball Hone...
2 Stroke Cylinder Walls wear differently than the Walls in a 4 Stroke. A regular Honing Stone will quickly reveal the Wear Areas around the Ports, while a Ball Home just camouflages that wear, besides messing up the Ports

 

[Mc Tool]

Yeah whatever you do dont go drilling holes in the piston ! Where the hell did that come from ?
Ultrasonic cleaners are not the magic bullet some peeps think they are for carbs . You will have to start poking thru small galleries and orifices to be sure they are clean . I use a nylon bristle of a broom or a bit of fluorocarbon leader , and flush with aerosol degreaser. You may have to remove some plugs to gain access to some passages ( this was recently covered in another thread ).......or you could just buy anew one off Ali ,I got one for my Tohatsu for 48 bucks NZ . I have given the carb the hairy eyeball and its the same as the original .

 

[keithb7]

I used a 3 finger, sprung, straight-style home. I performed a quick hone. Not over-doing it. The carb float was set as per factory service manual. The engine does not have a decompression system.

For break-in I warmed up the engine then worked it at various speeds. No full throttle for an hour. Up and down rpms, not staying at one speed for long periods. After a full hour of this I started to open it up to bursts of full throttle.

I have witnessed light through the main jet and its side tube holes after cleaning. I have sprayed compressed brake-clean through all orifices to check they are clear. I opened the carb bowl and hooked up fuel line. I watched the check valve open and close, alouwing snd stopping fuel entry when I manipulated the bowl.

I have synched the carb throttle lever with the timing advance cam. Engine kill switch is disengaged. Good spark is present. The black plastic venturi is present in the carb. I have tinkered with the pilot mixture screw, various settings. No improvement.

The one thing I keep going back to is 75 psi compression, and the fact that I re-used the pistons with new rings. The pistons looked good. There are no wear specs published. Cylinder taper and out-of-round measured great. I used both a snap-tee gage with micrometer, and a dial bore gage.

Would new pistons take the compression higher? I not sure but may try it just to solve this issue.

 

[jimmbo]

When you had the Engine apart, I will assume you cleaned the Ring Groves in the Piston. How did you do that?

What were the Piston to Cylinder Wall Clearances?
Did you put the Rings in the Cylinder, prior to installing them on the Pistons, and measure the Piston Ring Gap?

 

[Mc Tool]

Looks like you sure have been diligent. Unless the pistons are pretty rooted I dont think new ones will make much difference.
Im thinkin piston to bore clearance when new would be around 1.5 -2 thou and a wear limit of 3.5 -4 thou ( educated guess based on similar sized 2 stroke engines I am familiar with ) but I have seen an engine recently (65mm bore Rd400g ) with 8 thou piston clearance .......rattly as all hell but it still went ok .
I dunno what to rell you .....but that 75psi is hard to ignore .

 

[keithb7]

To clean the piston grooves I broke off a piece of the removed piston rings. I used it to scrape out any carbon. Honestly the engine was pretty clean in general when I tore it down. If extra scrubbing was needed I used Some steel wool and scotch-brite abrasive pads. Washed and blow-dried the pistons. Coated everything with 2 stroke oil upon assembly.

I did place the rings in the cylinders and measure end gap. At the top and bottom wear areas of the cylinder walls. I can’t recall the number now. I believe both cylinders measured within spec.

I did not measure piston to cylinders clearance. I suppose I should have. I’m negligent on that point. My thinking was it all looked pretty good. New rings, bearings and seals. Wham-bam…Should be good to go. (I’m a home hack hobby mechanic) Its pretty simple to pull the engine, tear it down and go back in. I may just do that when I have more time I have other outboard options to use this summer.

 

[keithb7]

I just studied my Mercury shop manual here again to confirm. There are no published tolerances for piston clearance. No compression specs. No piston measurement specs.

 

[Mc Tool]

Usually when I measure piston clearance its with new parts ( checking the clearance after a re bore ) anI usually just put a feeler gauge in the bore and if the piston still fits it has that much clearance . A bit rudimentary but it gives you a good indication .

 

[jimmbo]

Going back to the Original Problem of not being able to start without Copious amounts of Fuel being sprayed into the carbs
It does sound like a Fuel or Induction problem.
Are you using the Choke?
If I remember, the Fuel Pump is integral to the Carb, and there are 2 brands of Carbs used on the 9.8. Did you get the correct Overhaul Kit?
You replaced the Seals on the Crankshaft, what about all the Gaskets/Seals for the Crankcase?
In over 50yrs, I have only seen One Reed Leaf fail, so I am curious as to why you changed them?

 

[keithb7]

Are you using the Choke?
Yes. With or without the choke. Made no difference.

If I remember, the Fuel Pump is integral to the Carb, and there are 2 brands of Carbs used on the 9.8. Did you get the correct Overhaul Kit?

I think I had the right carb kit. I ran around on a local lake for 2 hours last Sat. Fuel pump seems to work fine. No issues with fuel delivery.

You replaced the Seals on the Crankshaft, what about all the Gaskets/Seals for the Crankcase?
In over 50yrs, I have only seen One Reed Leaf fail, so I am curious as to why you changed them?

Powerhead mounting gasket was replaced with new. Carb mount gasket replaced. I did not remove the cylinder head area cooling passage cover, nor exhaust cover. I did not disturb those, nor replace them. When I had the engine apart I inspected the reeds. I could see light through a couple them, in their relaxed position. With the case split and the crank out, there’s no better time. I figured I might as well remove any doubt there.

This thing is kicking my ass right now. Lol. Today it won’t start with a squirt of gas in the carb intake. Choke or not. Removed plugs. Strong spark is present. Cleaned plugs. Measured compression again. Just hair under 80 on the gage. Both cylinders. Engine kill switch is disconnected. Tried a squirt of raw (fresh) gas in the spark plug holes. No go. Won’t pop or fire with the compressed air fuel mixture in the cylinders. Too many pulls later, I have heard the air/fuel mixture ignite in what sounds like the exhaust/cooler area. Just a couple times. This leads me to think spark timing is off somehow. I measured resistance of ignition system components. Spark trigger measured high ohms. Tested good after replacing it with a spare from a ‘74 engine. As mentioned, good strong spark is seen when plugs are out and grounded to powerhead, and pull starter is yanked.

Next I think I’ll try putting the spare flywheel from the ‘74 on it. I see a cutline in the magnet on the inside of the ‘74 flywheel. I don’t see that same cut in the ‘79 flywheel. Curious, what causes the CDI trigger coil to collapse? A break in the inner flywheel magnet?