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Honda 8hp 4 stroke

david_rolt

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"Hi, Hope that someone can hel

"Hi, Hope that someone can help? I have an 8hp Honda 4 stroke that won't start, I have checked the following: - Compressions (cold eng) 80psi both cyl, plugs have a good consistant spark, stripped carb, all ok no dirt or blocked jets, and manual choke works fine. Have spun the engine over with an electric drill, (approx 1200RPM) strobe indicates plugs firing at approx 12 Deg BTDC. Have checked valve/cam timing, marks line up at TDC. Have pressurised cylinders with air line at TDC no sign of air at intake or exhaust, have turned engine till exhaust valves open and repeated test, good air bubbles appear in my test tank so seems exhaust not blocked. Have tried fresh fuel. Have removed carb and run engine over with drill and held a sheet of paper over the intake, it pulses back and forth, not sure if the back pulse is caused by valve overlap? Have removed inlet manifold and replaced gaskets. Plugs don't seem to be getting wet? Helllp"
 
"does primer bulb pump up firm

"does primer bulb pump up firmly?
if u are not getting wet plugs = no fuel
check to see if the vent lines are plugged or fuel lines
check fuel filter
pull the fuel line off the carbs and pump it, are u getting gas?

fuel pump is the last resort

try spraying starting fluid to see if it will ignite, u can only do this on a 4 stroke

pump up the cabs and see if gas gets in them by removing the bowl plug

could be your floats are set to low also, but i doubt that"
 
"Hi Just in time, thanks for y

"Hi Just in time, thanks for your reply, but carbs are my speciallity, so have checked and re checked lines etc, carb is getting fuel ok, and have tried the bowl plug for proof.
Primer bulb pumps up firm, and feul pressure to carb is 3.8psi at cranking. Haven't tried starting fluid yet but it is in my mind to give it a try. have even had the plugs out and dosed the cylinders with some fuel, still no sign of firing. Beats the hell out of me.

best regards

David"
 
"Hi Just in Time,

Purchased


"Hi Just in Time,

Purchased some Easy Start, sprayed it through the carb, even took plugs out and sprayed directly into cylinders, still no reponse whatsoever, This things has got to be either a mechanical problem, or the sparks not strong enough under compression? Spark works my strobe light ok.

Getting more grey hairs by the minute!

Reagrds

David"
 
"Hi Visitor,

Thanks for you


"Hi Visitor,

Thanks for your interest,

I have just tried your suggested test and the short answer is no, about 2.5 mm is the best it can make. I have a Selock manual, but it has no test spec for my ignition components. If anyone has some spec I would be greatful, the motor is a 1999 BF8A, single carb, rope starter.

regards

david Rolt."
 
"Hi Helms,

Any ideas as a s


"Hi Helms,

Any ideas as a source of test specs for the ignition system on the 8hp, I would natrally prefer to test not guess, i.e what voltage should I get at the primary connection of the ignition coil.

I think in this system the coil is triggered directly from the CDI unit, so if the capacitor within it does not reach full storage capacity, it may only trigger the coil primary with a low voltage, resulting in a low voltage collapse through the coil secondary windings,and a resulting low output, or does this sound like a load of tosh, say so if you think it I wont get upset! Oh for an ignition oscilloscope!

Thanks for your continues interest in my problems

regards

David"
 
If spark will only jump 2.5mm

If spark will only jump 2.5mm and no more it is not strong enough to fire a plug at compression.I do not have HONDA specs.I do know that parts are expensive.
 
u need a dva to test output of

u need a dva to test output of trigger and coil
100-300volts at coils
the trigger can be from 14v-200v
 
"Hi Just in Time,

Many than


"Hi Just in Time,

Many thanks for your reply,

It sounds like I should have between 100-300VDC at the coil primary, this is an orange wire that comes directly from my CDI unit.

With regards the CDI input trigger you refer to 14v-200v, my engine has the following coil arrangement: - A pulsar coil mounted directly below the cam gear wheel, I am guessing this tells the CDI the TDC position, and two other coils below the flywheel, one has two blue wires going directly into the rectifier, I am guessing this to be the system charge coil, so should not effect ignition output? the other has a single white wire, this connects to a brown wire going to the CDI box, is this the wire that should be between 14v-200v?

My Seloc workshop manual covers the engine very briefly and gives me no test procedure for any of the coils.

Thanks for your continued interest in my problem.

best regards

David"
 
"Hi Just in Time,

Have just


"Hi Just in Time,

Have just tested the primary voltage out from the CDI box. Whith the wire disconnected from the coil my DVM reads only 4.5vDC, guess this is the problem?

Regards

David"
 
u need to keep the coil connec

u need to keep the coil connected to get proper volts

the pulser is the trigger

what is the year?
 
"Hi Just in Time,

Will re c


"Hi Just in Time,

Will re check with coil connected and see what we get, thanks. The year is 1999.

I have just tested the Pulsar readings on a good running 9.9hp Honda, she only shows 1.1vAC but showed 11v > 16v DC? I know its a different eng but these readings seem strange to me, Was the 14v - 200v you stated in a prior posting AC or DC?

Best regards

David"
 
dc volts
the reason a asked w


dc volts
the reason a asked what year is that Honda changed some of their voltages this year
u can not get an accurate reading without a dva because of the voltage spikes
u also need test harnesses for the exciter/pulser and trigger tests
 
"Hi Just in Time,

Sorry tha


"Hi Just in Time,

Sorry that I haven't been back to you, I have been away.

I have located another 8hp Honda belonging to a friend, and its a good runner, so hope to borrow it, and will collect some data from that, and maybe transpose some parts, will let you know the outcome.

I am also persuing an oscilloscope, I am fed up with all this ohms data that never seems to read the same as the worshop manual, and yet the engines run, as we know ohms varies so much with temperature, and they never give you an ohms X temperature spec, so hopefully I will be able to collect and collate data from good and bad enginesin the future.

In the meantime, many thanks for your help

best regards

David."
 
"Hi Just In Time,

I managed


"Hi Just In Time,

I managed to borrow another 8hp from a friend, and collected a lot of running data from it, usual thing though, the readings were the same as the non running engine, so got his permission to transpose some parts. It turned out to be the igniter coil, although the readings from it were not far away from the non running engine. Bloody electrics drive you mad. Anyway using a spark gap testing tool that indicates approx KV and gap, you were right, a nice healthy 1/2" spark at 25KV output indication on the good engine, and only 5>7KV on the non running engine.

So thanks for you help, we just keep on learning new things everyday

Best regards

David"
 
"So lots and lots of people ma

"So lots and lots of people make the mistake of laying the plug on the block and saying "i have spark but the motor won,t start, must be the carburetor".Now you know better."
 
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