Bad Bog at WOT

Hey all, so I dug into my A-100 273 a bit, cleared up some problems, learned how to adjust valves, and now have a nice smooth idle. Sounds and starts better than it ever did. Yay!

However, I got a rebuilt carb, and put it on there, and it did clear up some of the idle problems but it created a new one: now I'm getting a really bad bog when quickly going from zero to WOT.

If I go slowly, it revs up no problem and sounds great. If I go quickly, it makes a bad noise, tries to stall, then surges up hard.

I think it's leaning out. It's got a new fuel pump and I'll replace the filter and check the fuel line, but it started doing this with the rebuilt carb. It's a 2-bbl stock style. Any help on what to adjust would be awesome. Timing should be good as well. Boo!

Parts store remans are notoriously bad. I've had awesome luck with these guys.

www.guaranteedcarbs.com You may need to call them since they don't go back past '71 on Mopars

Also check to make sure the timing is set properly and that the vacuum advance is working.

depending on age, the timing set will give you one helluva bog too. if you grab the fan blade, "not running of course ", and rotate the blade back and forth, you can feel the slack in the timing chain if its excessive. if the blade moves three or four inches before the load of the cams comes in, then that will add to your woes.
hmmmmmm 273 with adjustable valves equals 210 - 230 hp in stock form with all things good.

Go HEI. and leave 80% if your issues behind!!!

Thanks for the tips. With some tuning I've gotten it down to a stumble instead of a bog, but it's still not great. I'm going to keep tweaking it some and see if I can get it to be better. I think it's something to do with either the timing or the accelerator pump in the carb.

I swapped over to a Mopar electronic ignition, so points aren't a problem.

When I was adjusting the valves, and had the fan off anyways to make it easy to rotate the engine, I roughly tested the slack of the timing chain. While it wasn't perfect, it wasn't bad.

See, where I'm at with all this is trying to figure out if I can 'save' this engine, or if I should bite the bullet and get a rebuild. I'd like to save the money and drive on this engine for a year or two and then go for the rebuild...

Well Toast,
I'd say if you can get this one treaked enough to work and it doesn't smoke then use it. And work on the rest of the van and once you got everything else right you can always swap the motor later if need be.

A compression check will tell you alot about the condition of the engine. Wet and dry tests.

i've got the same exact issue with my 70 a-100... its a 225/6 with a single barrel i've checked/fixed everything i can think of and still no luck... im getting pretty mad at this thing.

donivan65 wrote:If you mean it bogs down at high RPM,,,,,,temporarily put a clear plastic fuel filter between the fuel pump and carb and watch the gas flow,,,,,if you see something happening in it, then you can have a restriction from the gas tank. look for other fuel filters in the line between the fuel pump and gad tank. Or you could have a clogged filter on the pick up tube in the tank.....so check this stuff out,,,,,,if the points are set right, it will act like this also,,,,,,

not high rpm, just if you give it more then a quarter throttle or try to climb a hill or any incline... i will try to list what i have checked just to rule out possible suggestions.

points and condenser are brand new and are set correct and have been triple checked. cap and rotor are very clean no heavy corrosion. coil is brand new cleaned and checked all connections. voltage to the coil spikes to 12 in crank and the comes back to around 8 volts at idle in park aprox 7.5 in drive. vaccum advance is working and the diaphram holds vaccum. checked the vaccum pod for the throttle on the carb and it is functioning and does hold vaccum. replaced the fuel filter (clear type) fuel pump, fuel lines and sending unit. carb has been rebuilt and all vaccum lines replaced. checked for vaccum leaks with carb cleaner and no results. i have gotten it to act up and richened the intake mixture to see if it was a lean condition with no luck, it did not make the problem better or worse.

there is zero vaccum at idle to the vaccum advance.. you cannot get the vehicle to act up in neatral or by holding the break and putting it under a load. there are no flat spots in the throttle at all when in park or neutral it just loses all power and boggs out when driving it up hill or just giving it more then a little gas to take off. the plug wires cap and rotor are not "new" but do not show any bad signs.. i have wet down the cap and plug wires to see if it would misfire or act up and they passed that test. yes the accelerator pump in the carb is working just fine. as far as the actual amount of advance from the vaccum advance i have not checked that yet but as far as i know that should not cause this type of problem unless it is way over advancing....

it will be noisey but try unhooking the exhaust at the manifold and go for another drive. possible restriction in the exhaust, not enough at idle to affect it but under bigger load........ worth a try.

jkr wrote:it will be noisey but try unhooking the exhaust at the manifold and go for another drive. possible restriction in the exhaust, not enough at idle to affect it but under bigger load........ worth a try.

i know the heat riser is working but i had not thought bout possible muffler restriction.... im just getting mad because i never throw parts at a problem i always narrow it down and plan my moves as i am always on a more then tight budget...

mice love to make nests in old exhaust pipes and mufflers. my snowmobile sits all summer with steel wool in the pipe to keep them out.

jkr wrote:mice love to make nests in old exhaust pipes and mufflers. my snowmobile sits all summer with steel wool in the pipe to keep them out.

very true and actually where i am located there are rats that come in the yard from the freeway located 50 feet behind my house.. one could have gotten in there but im thinking least likely scenario at this point.

Check your float level with a gauge and see if it is right. Sounds like a bad rebuilt carb to me.

Later,
Rip

RipVanArkie wrote:Check your float level with a gauge and see if it is right. Sounds like a bad rebuilt carb to me.

Later,
Rip

i have not done the final check after assembly through the top of the carb but i hav checked the initial reading of the float level with the bowl removed... only reason i do not think that is the case is if it were a lean condition when a gave it carb cleaner it should have helped it or made it worse if it was rich..

still no luck on this one boys.... im leaning towards a bad distributor as far as centrifugal weights etc. no slack in timing chain. have not checked for exhaust restriction but i kind of think that is a least likely scenario.. anybody got ideas i may be missing or over looking? its hard to want to work on something that runs like crap haha and fixing auto's is no new job for me. kinda stumped righ now on this one. again this is not a flat this is almost complete loss of power under load.

Hook up a vacuum gauge and drive it.
Rough numbers are 18" at idle in park, 16" in Drive, 18-20" cruising at 35,
3-7" brisk accelerating, over 20" decelerating, at higher speeds (freeway), the vacuum should hold steady, not slowly drop which would indicate the exhaust restriction.
A 273 2 bbl. would only see 0" when accel at idle, and should climb back up pretty quickly. Whacking the throttle in neutral from 1500 RPM or so should not drop below 3".
The BBD carb is notorious for flat spots; this can be worn throttle shafts, weak accel pump due to check ball seat worn, scored pump walls and worn pump rubber or leather. The passages can get clogged too. Another thing is the weak transition, which can be compenasted for by raising the main needles slightly.

One other thing I'd like to mention is that on older engines, carbon buildup on the back of the valves is common. Try using a cleaner product or have a service performed. I have had good luck with Seafoam (over the counter) and the BG fuel system cleaner (Pro service). It's is amazing the results I've seen especially on older stuff that isn't driven hard.

RodStRace wrote:Hook up a vacuum gauge and drive it.
Rough numbers are 18" at idle in park, 16" in Drive, 18-20" cruising at 35,
3-7" brisk accelerating, over 20" decelerating, at higher speeds (freeway), the vacuum should hold steady, not slowly drop which would indicate the exhaust restriction.
A 273 2 bbl. would only see 0" when accel at idle, and should climb back up pretty quickly. Whacking the throttle in neutral from 1500 RPM or so should not drop below 3".
The BBD carb is notorious for flat spots; this can be worn throttle shafts, weak accel pump due to check ball seat worn, scored pump walls and worn pump rubber or leather. The passages can get clogged too. Another thing is the weak transition, which can be compenasted for by raising the main needles slightly.

One other thing I'd like to mention is that on older engines, carbon buildup on the back of the valves is common. Try using a cleaner product or have a service performed. I have had good luck with Seafoam (over the counter) and the BG fuel system cleaner (Pro service). It's is amazing the results I've seen especially on older stuff that isn't driven hard.

ok i'll hook up a vac gauge and see what we got after i get the radiator back in it this weekend. i rebuilt the carb myself (not my first) cleaned and blew out all circuits/ passages, inspected everything and checked the float level. the van was running the same before and after so unless it is a worn piece of the carb like you mentioned i dont think it involved anything i did to the carb. im familiar with seafome and can give it a try to although i've never seen it cure a prob like this. just a note: i'll list all the parts i have replaced so far. some because they were not working at all and some just to try and isolate the problem. i have replaced: sending unit in tank, all fuel lines, fuel filter, fuel pump, (dead when i got the van) points and condenser, spark plugs, coil, (cleaned up and changed conections at coil they were old and falling apart) rebuilt carb, then took back apart and double checked it, set timing at factory spec. checked vac advance and is working and holding vac, changed vac lines (only one on engine) have run several tank fulls of fresh gas, also tried advancing and retarding the timing to see the effects? (none)