Hole shots, engine backfiring, stumbling - timing?
John Jackson
Hello:
I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
supposed to be 8 degrees BTDC. If I set it to that, the engine often
stumbles, backfires through the carb or dies on hole shots to get
skiers up. Setting the timing to 11 degrees BTDC almost but not quite
completely cures the trouble. At that setting, most hole shots are
strong and smooth but occasionally I'll get the backfire through the
carb. This usually happens if I idle the engine for an extra minute or
two while talking to a skier in the water before pulling them up again.
Another possible clue is that while taking a fun buzz across the
lake at wide open throttle, I tried sharply bringing the throttle back to an
idle for a second and then sharply opening it up wide again. Upon
opening the throttle like this, the engine lagged/bogged noteceably for
a second or so before returning to full power/RPM's.
The engine has 193 hours and has been well cared for, including oil
changes every 40 hours. Other than the above trouble, it runs powerfully
and smoothly. I don't suspect any internal engine damage or wear as the
source of this problem, ie., jumped/stretched timing chain, burned
valves, poor compression, etc. I'm running 93 octane gas.
I'm thinking that I might either be having a problem with my
accellerator pump or, more likely, the mechanical advance in the
distributor. When I put the timing light on the engine, it takes
somewhat more than just a small hit on the throttle to get the
mechanical advance to begin to shift the position of the timing mark
noticeably. Is this correct behavior or should it begin to move
noticeably as soon as engine RPM's begin to increase from an idle (~900
RPM's)?
Does it sound like I am on the right track here? Can anyone think
of anything else I should be considering? Thanks for all your help!
Don Merritt
John Jackson <jo...@asimov.oit.umass.edu> wrote in article
<501t05$8...@nic.umass.edu>...
> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
> supposed to be 8 degrees BTDC. If I set it to that, the engine often
> Does it sound like I am on the right track here? Can anyone think
> of anything else I should be considering? Thanks for all your help!
I expect that you will get a lot of theories here, so add this one to the
list. I believe that your problem could be almost anything from
Carburation to Timing.
I had a similar experience with my Electronic ignition PCM a couple of
years ago, and replaced all sorts of stuff before a crack mechanic
diagnosed the problem. The only difference between your description and my
problem was that mine was only periodic (<15% of the time).
My problem ended up being a loose ground wire on the electronic ignition
box. We took the whole thing appart, coil by coil (substituting in a known
good coil pack), and I haven't had the problem since.
Good luck.
Theo Freund
In article <01bb961e$8521f100$3c19...@stealth.iinet.net.au> "Don Merritt" <Do...@merak.com> writes:
>John Jackson <jo...@asimov.oit.umass.edu> wrote in article
>> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
>> supposed to be 8 degrees BTDC. If I set it to that, the engine often
>> Does it sound like I am on the right track here? Can anyone think
>> of anything else I should be considering? Thanks for all your help!
>
>I expect that you will get a lot of theories here, so add this one to the
>list. I believe that your problem could be almost anything from
>Carburation to Timing.
Try this one. You should be able to answer this one right away.
Make sure your distributor is not cracked and not damp on the inside.
(if you have a newer engine w/o a distributor caps then ... never mind :).
--Theo
____________________________________________________________________________
Theo Freund Internet: th...@amcu-tx.sps.mot.com
Motorola, Semiconductor Products Sector X.400: RZV...@email.sps.mot.com
AMCU - Modular Microcontroller Design Phone: (512) 891-4419
tofer
In article <501t05$8...@nic.umass.edu>,
jo...@asimov.oit.umass.edu says...
> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory
>>snip<<
> Does it sound like I am on the right track here?
Sounds like the carb is suspect. Check the accelerator pump and
nozzle. What year is it?
Tzilg
Do you have electronic ignition or distributor? Could be the mechanical
advance in the distributor or the carb, but most likely is one of those
two things. Have a compression test done first that doesn't cost anything,
then check the distributor, then go for the carb. Good luck.
Vincent Young
John Jackson wrote:
>
> Hello:
>
> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
> supposed to be 8 degrees BTDC. If I set it to that, the engine often
> skiers up. Setting the timing to 11 degrees BTDC almost but not quite
> completely cures the trouble. At that setting, most hole shots are
> strong and smooth but occasionally I'll get the backfire through the
> carb. This usually happens if I idle the engine for an extra minute or
> two while talking to a skier in the water before pulling them up again.
> Another possible clue is that while taking a fun buzz across the
> lake at wide open throttle, I tried sharply bringing the throttle back to an
> idle for a second and then sharply opening it up wide again. Upon
> opening the throttle like this, the engine lagged/bogged noteceably for
> a second or so before returning to full power/RPM's.
> The engine has 193 hours and has been well cared for, including oil
> changes every 40 hours. Other than the above trouble, it runs powerfully
> and smoothly. I don't suspect any internal engine damage or wear as the
> source of this problem, ie., jumped/stretched timing chain, burned
> valves, poor compression, etc. I'm running 93 octane gas.
> I'm thinking that I might either be having a problem with my
> accellerator pump or, more likely, the mechanical advance in the
> distributor. When I put the timing light on the engine, it takes
> somewhat more than just a small hit on the throttle to get the
> mechanical advance to begin to shift the position of the timing mark
> noticeably. Is this correct behavior or should it begin to move
> noticeably as soon as engine RPM's begin to increase from an idle (~900
> RPM's)?
> of anything else I should be considering? Thanks for all your help!
>
> jo...@external.umass.edu
John, You've got a friction problem on the mechanical advance. I'll be
the guts of your distibutor are original. Try replacing everything
under the cap (and that, too, if you've got an extra 14 bucks). The
advance springs may be too tight for your taste, in which case you need
to look to mallory or holley or accell for some mod springs. And, you
can't be too careful with point gaps and alignment. There's a science
to gapping points that far exceeds the pocket knife-business card method
you'll see some amateurs use on spark plugs.
Remember that you want your guts in the 1500-3000 rpm range for skier
hole shot. Car engine thinking will almost always push you into the
2200-4500 rpm range, looking for a top end at 5200-5500. That's way too
high for your boat unless you happen to have a 1:1 ratio outdrive
(unlikely)
These symptoms dissapeared after I got my 260 running right.
Theo Freund
In article <324832...@r07.tdh.tx.state.us> vyo...@r07.tdh.tx.state.us writes:
>John Jackson wrote:
>>
>> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
>> supposed to be 8 degrees BTDC. If I set it to that, the engine often
>> -John
>> jo...@external.umass.edu
>
>John, You've got a friction problem on the mechanical advance. I'll be
>the guts of your distibutor are original. Try replacing everything
>under the cap (and that, too, if you've got an extra 14 bucks). The
>advance springs may be too tight for your taste, in which case you need
>to look to mallory or holley or accell for some mod springs. And, you
>can't be too careful with point gaps and alignment. There's a science
>to gapping points that far exceeds the pocket knife-business card method
>you'll see some amateurs use on spark plugs.
I think a dwell tackometer (sp) will allow you to set the points
properly. (I forget if dwell can be set separate from point gap -
don't have to set it any more.) Actually what I very highly recommend
is that you install an electronic conversion kit (under $100). My old
351 PCM engine runs SOOO much smoother at idle and better in general
with it. I think the kit comes with new advance springs. Call
Discount Inboard Marine. They will know if they have the right kit and
what it contains.
>Remember that you want your guts in the 1500-3000 rpm range for skier
>hole shot. Car engine thinking will almost always push you into the
>2200-4500 rpm range, looking for a top end at 5200-5500. That's way too
True. In my Nautique, top end is at 4400 RPMs with a 13R13 O.J. prop
(standard pitch for this boat but slightly more cupped then the
original Michigan Propeller). I'm pulling slalom skiers at 2500-3500
RPM's (forget specific RPM for a given speed).
Rod Mc Innis
Theo Freund (th...@waterski.sps.mot.com) wrote:
: I think a dwell tackometer (sp) will allow you to set the points
: properly. (I forget if dwell can be set separate from point gap -
: don't have to set it any more.)
Dwell is a measure of the ratio of point open to point closed time.
Setting the gap sets the dwell. Some distributors have a little door
that can be opened and points that can be adjusted while the engine is
running. On these distributors, setting the dwell (gap) is trivial.
Unfortunately, this feature seems to be rather rare, which means
that you need to pop the cap to set the points. The dwell meter is
really only useful for checking the results and making sure that it
hasn't moved.
For an 8 cylinder engine, a dwell angle of 28 to 32 degrees is
generally accepted as acceptable.
To the orignal poster: One thin that you may want to do is make
sure that the timing mark pointer is really in the right spot, and is
for the proper rotation engine.
There was an engine I worked on once that I spent days trying to
make run right, and then finally discovered that the little marker
that was used to reference the timing marks had been put on under the
wrong bolt.
You should also be sure that the "BTC" marks go by before the "ATC"
marks. It would be a long shot, but it is possible that timing marks
were meant for the opposite rotation engine. And lest you think that
opposite rotation engine are so rare that you don't need to worry
about it, you should realize that a fair percentage of inboards, like
the tournament style boats, use an engine that rotates opposite of the
standard automotive application.
Rod McInnis
wcv...@superlink.net
Vincent Young <vyo...@r07.tdh.tx.state.us> wrote:
>John Jackson wrote:
>>
>> Hello:
>>
>> I have a Mercruiser 260 (Chevy 350/5.7L V8). Factory timing is
>> supposed to be 8 degrees BTDC. If I set it to that, the engine often
>> -John
>> jo...@external.umass.edu
>
>John, You've got a friction problem on the mechanical advance. I'll be
>the guts of your distibutor are original. Try replacing everything
>under the cap (and that, too, if you've got an extra 14 bucks). The
>advance springs may be too tight for your taste, in which case you need
>to look to mallory or holley or accell for some mod springs. And, you
>can't be too careful with point gaps and alignment. There's a science
>to gapping points that far exceeds the pocket knife-business card method
>you'll see some amateurs use on spark plugs.
>Remember that you want your guts in the 1500-3000 rpm range for skier
>hole shot. Car engine thinking will almost always push you into the
>2200-4500 rpm range, looking for a top end at 5200-5500. That's way too
>high for your boat unless you happen to have a 1:1 ratio outdrive
>(unlikely)
>These symptoms dissapeared after I got my 260 running right.
Check the linkage on your carburetor- the link from the vaccum
diaphragm at the right front corner of the carb. to the secondary air
valve on many of these carbs. is supposed to be tight (.015 clearance)
when the diaphragm is seated by vacuum. This is because these carbs.
incorporate two secondary booster wells in the carburetor body which
open into the secondary venturis immediately above the closed air
valve; if these are exposed to the lower (vacuum) side of the venturi
before the air valve is released by the vacuum diaphragm, the proper
enrichment is not available at the beginning of the secondary opening
cycle and the engine (which in a boat is always under heavy load)
stumbles or bogs for 1-3 seconds. Further, once you adjust the
clearance to .015" with the diaphragm seated, be sure that the
secondary air valves will fully open with the diaphragm "relaxed."
If they will not ( they normally just barely make it against the
linkage) replace the vacuum diphragm - the washers that hold the
rubber diaphragm to the operating rod (inside the little plastic case)
have rusted and reduced its overall range.