My 1970 marine 225 idles fine but if in gear it stumbles and sometime backfires when the throttle is advanced. I can sort-a feather it past this rough spot and get the motor and boat up to full speed. In neutral it revs nicely.

I set the idle and mixture screws as suggested by SSD and the timing is set at 5 BTDC (it acted the same at 11 BTDC). The timing seems to advance but I only checked the initial bit of advance that I could cause by slightly advancing the throttle by putty pressure in the linkage. There is no vacuum advance.
The plugs are newer and gaped, all are dry but have dry black carbon dust (#5 might be a little wet).
The carb has been rebuilt (carter 2 barrel) and the accelerator pump shot seems robust, the linkage is adjusted. I sprayed an oil product around the base of the carb. while the boat was idling but I could not detect any variation in how the motor ran.

the points are new and set and the dwell seems steady.
Also I have set the valve lash as per these instructions from SSD
http://www.slantsix.org/forum/viewtopic ... ght=#72139

The gas is fresh and plenty of fuel seems to be is getting to the carb.

I have not idea what to do. Please help because I really love this boat.

I have searched and read many of the articles about stumbling motors on this web site but I am at my wits end.

Also, is this item real? http://www.ebay.com/itm/Carter-BBD-lowt ... es&vxp=mtr

also, does anyone know of any of those old Chrysler training films that are about the marine six?

One last question. This boat has a water pump that runs by pulley, but I think that not enough water moves through the motor because there seems to be a hot spot on the very front of the exhaust which is the point where water coming from the front of the motor is routed to the exhaust manifold.....anyone know anything about the marine 6? Also, there is a thermostat that can route water either to the cab. base or the exhaust elbow. Where is water sent when the motor is warmed up? To the carb. base or the exhaust elbow?
The motor has a water jacketed open exhaust that terminates just below the water line at idle so it sounds like a tractor (in a good way).

Thank you very much for your help.

A stumble as described can be fuel related, basically a lean condition, caused by a late transition from the carb's idle circuit, into main feed or the "power" circuit. ( a vacuum gauge will help find & "trouble-shoot" this issue)

Another cause is in the distributor, missing or late spark advance.
The mechanical advance system can be sticky or frozen-up... a timing light can help you "see" that. Does the distributor pull-in lots of mechanical advance, right off idle and does it quickly get to 30 degrees of total spark advance?
DD

wlockwood, hi , your boat needs a fatter shot of fuel from the pump jet with 2012 gas specs , not 1970"s. you have mabe a .025-.027 pump shot . we will help you get to a .028 to .032. for a boat ,you always need a FAT squirt to get out of the hole. post back, ron

All good input, I second the pump shot being week and mechanical advance being sticky. The pump shot hole can corrode over time and reduce the hole size and the grease can get gummy on the mechanical advance plate. Hopefully the power valve circuit is not plugged and is working properly. Look in the bottom of the float bowl to see if there is any water droplets moving around and covering up the pwer valve jet or the main jets. That can cause havoc........hopefully you are running a fuel water separator between the tank and carb.

Let us know what the total advance is and at what rpm it's all in. Have some one else drive the boat, give it a squirt of extra fuel if you need to get it on a plane, then watch the timing light. All 30 degrees should be in by 2000 rpm.

When I used to work on these engines, 4 to 6 degrees initial advance worked best.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

The carb you linked is not "real" -- it is a Chinese knockoff. These are of extremely poor quality, to the extent that I would call them seriously dangerous (fire hazard).

One of the clues that you should run away is when a new Carter BBD carburetor is described as "Low top" or "high top" in the ad/auction. This is not 100% reliable because the phony carburetors are not always described that way.

Also, even if it were a real carburetor, it is not a marine item.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thank you all for the advice, these all sound right on and this gives me hope.
I have a vacuum gauge that has never been used. How do I use the gauge to diagnose?

It sounds like the change in the amount of vacuum causes the carb. to transition from idle circuit to the main feed. So if the change in vacuum is normal when throttle is advanced but the carb. is slow to respond then the carb. has a problem. Assuming this true, what needs to happen to fix? I will look at the carb. schematic and attempt to ascertain how that part of the carb. works. But please anyone feel free to chime in with an explanation of how this part of this carb. works.


As for the spark advance. It seemed to pull some advance right away with only a small increase in RPM that I could cause by leaning on the linkage. I need to check this more carefully.

Just for my mental clarity on timing; my timing at idle is at 5 BTDC and it should advance 30 degrees by the time the motor achieves 2000 rpm.
This advance of timing causes the spark to arrive "later" (ATDC) when the piston is on its way down?
And if the advance is lagging I should pull the distributor and inspect and clean, reinsert it the exact position and re-check.

If a "fatter" shot of fuel from the pump jet is needed because gas specs have changed...
How do I change this?
I will be checking the pump shot hole and that area and will post back.

Again, many thanks.

CLICK ME
No, the change in the angle of the throttle blades causes the carb to transition from the idle circuit, to the transfer circuit, and then finally to the main circuit. As the throttle is opened, different holes in the throat of the carb are exposed to engine vacuum. These holes correspond to various circuits in the carb. I highly recommend you read the appropriate carb operation and repair manual linked HERE. You want the Carter BBD manual.
It is possible the carb has a problem, but there could be other issues. Read the vacuum gauge article I linked to and read the carb operation manual I linked to.
Timing advance is controlled by two mechanisms: vacuum advance and mechanical advance. The vacuum advance does not operate when the throttle is closed. This is because no vacuum is applied to the vacuum advance can when the throttle is closed. As soon, or almost as soon, as the throttle is opened, vacuum is applied to the vacuum advance can and the timing is advance. You should be aware that not all vacuum advance cans are identical. Different cans provide different amounts of advance and apply it at different vacuum levels. Do a search of this site for posts about "distributor recurving" for many full and in-depth discussions of this topic. It is also possible for the diaphragm in the distributor to degrade and rupture with age, so opening the throttle creates a big vacuum leak. You need to verify that the vacuum advance can still holds a vacuum.

The mechanical advance is controlled by two weights and springs whizzing around below the breaker plate. Centrifugal force causes the weights to move outward on their pivots, which in turn causes the breaker plate to turn and advances the timing. You can fine tune the mechanical advance by adjusting the springs. Again, search for threads discussing distributor recurving.
Base timing of 5BTDC is about normal, but advancing the timing makes the spark fire earlier, or more degrees BTDC. The rule of thumb for slant sixes is to have the mechanical advance set up so that the total advance (mechanical plus base) is at 30 BTDC by 2000-2500 RPM. Vacuum advance will add another15-20 degrees of advance at light throttle cruise, so the total timing advance (base + mechanical+vacuum) will be somewhere around 50 BTDC.

Timing advance curve is a complicated topic and I highly recommend you read the threads about distributor recurving. It took me about a year before I fully grasped all the different concepts and understood them.
Absolutely. CHeck that the vacuum advance can holds a vacuum and clean and lubricate the springs and weights. Also, verify that the distributor is hooked to a port on the carb that has no vacuum at idle, but full manifold vacuum when the throttle is opened.
On a BBD the only adjustment that can be made to the pump shot is to switch holes or bend the linkage for a longer or shorter shot. There is no pump jet or cam or nozzle that can be changed out.

One adjustment people often forget on the BBD is the metering rod adjustment. This has a large impact on how the carb operates. Read the operation manual linked above.
That's why we are here! And if you get a chance, please post some pictures. We don't see many slant six powered boats.

Which Carter BBD do you have? The old air metered units have some adjustment with the Allen screw on the metering rod tree. The newer ones with the sheet metal cover have more adjustment.

Apples and oranges.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

No vacuum advance on marine distributors; no port for vacuum advance on marine carburetors.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Whoops. Thanks for correcting that Dan.

What type of vacuum advance do mari................

But, what type of vacuum advance is on the space shuttle 225's?

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

I'm late to the (actual-question) party here, sorry 'bout that, but it does sound as if the carburetor's overdue for some (more) attention. What was the nature of the rebuild job? If it was just a pull/clean/reassemble-with-new-gaskets, it may well need more.

When's the last time the valves were adjusted?

Consider a service manual for your engine as a mandatory tool. You can buy it here (search page for MH-225D and you'll find the right manual).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

The carb is a manually choked Carter BBD 4268S (or maybe a 3389S). The boat is a 1970 the motor is designated a M-225-D. Correct Craft tells me the motor was placed into the boat on 9/25/1969.
According to this http://www.tocmp.com/manuals/Carbs/Cart ... ndMisc.pdf
The 3389S is dated 1962-1965, the 4268S is dated 1965-1971.
Both numbers pull up the same rebuilt kit on the Mike's Carb. Parts site.

Thanks for the direction. Upon further study today I am betting that my problem is float level adjustment is too restrictive. This can reduce the pump shot and probably reduce the transfer port richness. I think I said that right.

I have had the carb. apart several times in the past but I have never properly adjusted the float level...any advice for this adjustment? And I vaguely remember messing with tab...this I should have mentioned in the history but I had forgotten about this.

I have not watched all the movies yet or read all the manual yet...the info I need to adjust the float level is probably there.

I am away from the boat for a few days I will try to post some pictures and will post an update after I get a chance to work on the carb.

Thank you for the advice.

Dan,
The carb. was sent to Georgia for an expensive rebuilt...not sure they did much more than clean and change gaskets. After I installed it I notice fuel leaking. I discovered that the "top plate" screws where loose. This sort-a made me lose confidence in the work. I sent it there because they seemed best place and because I lost the ID tag so I had trouble getting the right kit and I thought it might need some re-bushing.

The good news is that after reading and watching just part of the data that people have pointed me towards I have figured out the ID number and found rebuilt kit on "Mike's Parts" site and most importantly learned a huge amount about the mysteries of how carbs. work. Thus I am ready to do this.

Question: if my carb. needs a re-bush how do I know and who do I get to do it?
Valves adjusted just a few days ago, just before I gave up hope.
Question: About the book (which I really need) are you sure about MH-225D? My motor is designated M-2225D which is the book below that. Too bad they don't list the years.

The book I pointed at is appropriate for the year range that covers your engine. The book next to it is for earlier engines. The "H" is for the SpaceSaver (tilted the rest of the way over on its side) version of the engine and the book I pointed at seems to cover both the H and the non-H versions.

Which outfit (name) did you send the carb to for rebuild? I can think of one place in Georgia on my "probably trustworthy" list.

Float settings aren't "too restrictive"; it's either too low or too high. A low float will cause lean running, which will cause stumble and spitback on attempted acceleration. Checking requires the proper gauge tool, which unfortunately is no longer included in carburetor repair kits available from mass-market sources (those kits, by the way, are of poor quality any more and aren't worth messing with) nor even from medium-level specialists like Daytona Parts Co (whose inlet needle/seat design I like best). I'm not sure if the much more extensive (and expensive) kit from www.thecarburetorshop.com would contain the float gauge or if perhaps they'd send you one separately. The BBD manual has float check-and-set procedures. You must use a new airhorn-to-body gasket each and every time you separate these two castings.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.